Systems and methods for wavelength and intensity referencing

Abstract

Systems and methods discussed herein may include a wavelength reference module. The wavelength reference module may include at least one coarse wavelength reference configured to receive an electromagnetic signal, the coarse wavelength reference being configured to filter the electromagnetic signal; and at least one coarse wavelength detector configured to measure the intensity of at least a portion of the filtered electromagnetic signal. Systems and methods discussed herein may further include a fine wavelength reference. Systems and methods discussed herein may further include a calibration module.

Description

Systems and Methods for Wavelength and Intensity ReferencingRELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 617,700, filed on January 4, 2024. The entire teachings of the above application are incorporated herein by reference.BACKGROUND

[0002] Wavelength jitter (i.e., unwanted shifts in wavelength) in an electromagnetic signal may be a limiting factor for imaging, scattering, and spectroscopy applications. When an electromagnetic signal filters through an etalon the transmitted intensity varies as a function of wavelength. Deviations in this etalon spectrum can be used to characterize wavelength jitter; however, it is challenging to detect wavelength shifts larger than half of the free spectral range of the etalon and challenging to characterize wavelength jitter between intensity peaks.SUMMARY

[0003] Aspects of inventive concepts relate to a reference system that comprises a wavelength reference module. The wavelength reference module may comprise at least one coarse wavelength reference configured to receive an electromagnetic signal, the coarse wavelength reference being configured to filter the electromagnetic signal; and at least one coarse wavelength detector configured to measure the intensity of at least a portion of the filtered electromagnetic signal.

[0004] In various example embodiments, the electromagnetic signal comprises optical wavelengths.

[0005] In various example embodiments, the electromagnetic signal comprises infrared wavelengths.

[0006] In various example embodiments, the electromagnetic signal comprises ultraviolet wavelengths.

[0007] In various example embodiments, the electromagnetic signal is generated by a laser.

[0008] In various example embodiments, the electromagnetic signal is generated by a tunable laser.

[0009] In various example embodiments, the wavelength reference module comprises a coarse wavelength beam director configured to receive an electromagnetic signal from an electromagnetic source and redirect a portion of the electromagnetic signal towards the at least one coarse wavelength reference.

[0010] In various example embodiments, the wavelength reference module comprises a coarse wavelength beam director configured to receive an electromagnetic signal from an electromagnetic source and transmit a portion of the electromagnetic signal towards the at least one coarse wavelength reference.

[0011] In various example embodiments, the coarse wavelength beam director directs a portion of the electromagnetic signal in a direction approximately orthogonal to the incident direction.

[0012] In various example embodiments, the coarse wavelength beam director comprises a beam splitter.

[0013] In various example embodiments, the coarse wavelength beam director comprises a mirror.

[0014] In various example embodiments, at least one of the at least one coarse wavelength reference comprises an etalon.

[0015] In various example embodiments, the coarse wavelength reference comprises more than one layer.

[0016] In various example embodiments, each layer of the at least one coarse wavelength reference comprises a different material.

[0017] In various example embodiments, one or more layers of at least one of the at least one coarse wavelength reference comprises the same material.

[0018] In various example embodiments, the coarse wavelength reference comprises four layers.

[0019] In various example embodiments, at least one of the at least one coarse wavelength reference comprises a layer of sapphire, a layer of silicon dioxide, and two layers of silicon.

[0020] In various example embodiments, at least one of the at least one coarse wavelength reference comprises a layer of sapphire directly above a first layer of silicon, thefirst layer of silicon being directly able a layer of silicon dioxide, the layer of silicon dioxide being directly above a second layer of silicon.

[0021] In various example embodiments, at least one of the at least one coarse wavelength reference comprises a first (bottom) layer, a second layer, a third layer, and a fourth (top) layer.

[0022] In various example embodiments, the first layer and the third layer comprise the same material.

[0023] In various example embodiments, one or more layers of at least one of the at least one coarse wavelength reference comprises the same thickness.

[0024] In various example embodiments, the fourth layer is thicker than the other layers.

[0025] In various example embodiments, at least one of the at least one coarse wavelength reference comprises at least one angled surface.

[0026] In various example embodiments, at least one of the at least one coarse wavelength reference comprises a coating.

[0027] In various example embodiments, at least one of the at least one coarse wavelength reference comprises a reflective coating.

[0028] In various example embodiments, one or more layers of at least one of the at least one coarse wavelength reference comprises one or more coatings.

[0029] In various example embodiments, one or more layers of at least one of the at least one coarse wavelength reference comprises one or more reflective coatings.

[0030] In various example embodiments, the intensity measured at the at least one coarse wavelength reference detector is normalized to the electromagnetic signal intensity.

[0031] In various example embodiments, the reference system further comprises an input polarizer constructed and arranged to polarize the electromagnetic signal that is received by the at least one coarse wavelength reference.

[0032] In various example embodiments, at least one of the at least one coarse wavelength reference is configured such that a transmitted intensity is monotonically increasing over a wavelength range of 2100 nm to 2400 nm.

[0033] In various example embodiments, the reference system further comprises an intensity reference module. The intensity reference module may comprise at least one intensity reference module detector configured to measure the intensity of at least a portion of the electromagnetic signal; and at least one intensity reference beam director configured toredirect the at least a portion of the electromagnetic signal towards the at least one intensity reference module detector and pass a portion of the electromagnetic signal to the sample.

[0034] In various example embodiments, the intensity reference beam director directs a portion of the electromagnetic signal in a direction approximately orthogonal to the incident direction.

[0035] In various example embodiments, at least a portion of the electromagnetic signal transmits through the at least one coarse reference beam director and propagates to the intensity reference module.

[0036] In various example embodiments, at least one coarse wavelength beam director is configured to alternate between directing the electromagnetic signal to the wavelength reference module and directing the electromagnetic signal to the intensity reference module.

[0037] In various example embodiments, the at least one input polarizer is positioned between the electromagnetic source and the intensity reference module.

[0038] In various example embodiments, the reference system further comprises a fine wavelength reference module. The fine wavelength reference module may comprise at least one fine wavelength beam director; at least one fine wavelength reference configured to receive a first portion of the electromagnetic signal at a first incident angle and second portion of the electromagnetic signal at a second incident angle, the fine wavelength reference being configured to filter the first portion of the electromagnetic signal and the second portion of the electromagnetic signal; a first fine wavelength detector configured to measure the intensity of at least a portion of the filtered first portion of the electromagnetic signal; and a second fine wavelength detector configured to measure the intensity of at least a portion of the filtered second portion of the electromagnetic signal.

[0039] In various example embodiments, the fine wavelength beam director comprises a beam splitter.

[0040] In various example embodiments, the fine wavelength beam director comprises a mirror.

[0041] In various example embodiments, the at least one fine wavelength beam director comprises a first fine wavelength beam director configured to direct the first portion of the electromagnetic signal to the at least one fine wavelength reference and a second fine wavelength beam director configured to direct the second portion of the electromagnetic signal to the at least one fine wavelength reference.

[0042] In various example embodiments, the wavelength reference module further comprises an intermediate beam director configured to steer at least a portion of the electromagnetic signal to the fine wavelength reference module.

[0043] In various example embodiments, the intermediate beam director is positioned between at least one of the at least one coarse wavelength beam director and at least one of the at least one coarse wavelength reference.

[0044] In various example embodiments, the fine wavelength reference comprises an etalon.

[0045] In various example embodiments, the reference system further comprises a calibration module. The calibration module may comprise at least one calibration reference configured to receive a portion of the electromagnetic signal, the calibration reference being configured to filter the first portion of the electromagnetic signal and the second portion of the electromagnetic signal; at least one calibration beam director configured to direct a portion of the electromagnetic signal to the at least one calibration reference; and at least one calibration detector configured to measure the intensity of at least a portion of the filtered electromagnetic signal.

[0046] In various example embodiments, the wavelength reference module comprises an intermediate beam director configured to steer at least a portion of the electromagnetic signal to the calibration module.

[0047] In various example embodiments, the intermediate beam director is positioned between at least one of the at least one coarse wavelength beam director and at least one of the at least one coarse wavelength reference.

[0048] In various example embodiments, the calibration module comprises a first calibration beam director configured to direct a first portion of the electromagnetic signal to the calibration reference a second calibration beam director configured to direct a second portion of the electromagnetic signal to the calibration reference.

[0049] In various example embodiments, the calibration module comprises at least one calibration beam director that is configured to rotate so that it can direct a portion of the electromagnetic signal towards the calibration reference at different incident angles.

[0050] In various example embodiments, the calibration reference comprises an etalon.

[0051] In various example embodiments, the calibration reference includes an etalon comprising a thickness of 100 microns.

[0052] In various embodiments, the calibration reference comprises more than one layer.

[0053] Aspects of inventive concepts relate to a method of referencing an electromagnetic signal. The method may comprise providing a reference system. The reference system may comprise a wavelength reference module. The wavelength reference module may comprise at least one coarse wavelength reference configured to receive an electromagnetic signal and at least one coarse wavelength detector. The method may also comprise filtering the electromagnetic signal through the at least one coarse wavelength reference; and measuring the intensity of at least a portion of the electromagnetic signal that filters through the coarse wavelength reference at the at least one coarse wavelength detector.

[0054] In various example embodiments, the method further comprises normalizing the intensity measured at the at least one coarse wavelength reference detector to the electromagnetic signal intensity.

[0055] In various example embodiments, the method further comprises adjusting the wavelength of the electromagnetic signal between 2100 nm to 2400 nm.

[0056] In various example embodiments, at least one of the at least one coarse wavelength reference is configured such that the transmitted intensity is monotonically increasing over a wavelength range of 2100 nm to 2400 nm.

[0057] In various example embodiments, the method further comprises providing a fine wavelength reference module. The fine wavelength reference module may comprise at least one fine wavelength reference; a first fine wavelength detector; and a second fine wavelength detector. The method may further comprise filtering a first portion of the electromagnetic signal though at least one of the at least one fine wavelength reference at a first incident angle; filtering a second portion of the electromagnetic signal though the at least one of the at least one fine wavelength reference at a second incident angle; measuring the intensity of the first portion of the electromagnetic signal that filters through the fine wavelength reference at the first incident angle at the first fine wavelength detector; and measuring the intensity of the second portion of the electromagnetic signal that filters through the fine wavelength reference at the second incident angle at the second fine wavelength detector.

[0058] In various example embodiments, the method further comprises providing at least one fine wavelength beam director; and directing a portion of the electromagnetic signal to at least of the at least one fine wavelength beam director.

[0059] In various example embodiments, the method further comprises providing a calibration module. The calibration module may comprise at least one calibration reference; and at least one calibration detector. The method may comprise filtering a portion of the electromagnetic signal through the at least one calibration reference; and measuring an intensity of at least a portion of the electromagnetic signal that filters through the calibration reference at the at least one calibration detector.

[0060] In various example embodiments, the at least one coarse wavelength reference comprises an absorptive element with a monotonic response to changes in the wavelength of the electromagnetic signal.

[0061] In various example embodiments, the at least one coarse wavelength reference comprises glass.

[0062] In various example embodiments, the at least one coarse wavelength reference comprises two layers of silicon separated by a spacer to form a stable etalon.BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.

[0064] FIG. 1 shows an example embodiment of a reference system, in accordance with aspects of inventive concepts.

[0065] FIG. 2A shows electromagnetic radiation filtering through an example embodiment of an etalon and towards a detector, in accordance with aspects of inventive concepts.

[0066] FIG. 2B is an example graph of a modeled transmission intensity as a function of wavelength at a fixed position for electromagnetic radiation filtering through an etalon comprising 20 microns of BK7 glass.

[0067] FIG. 3A shows an example embodiment of a coarse wavelength reference, in accordance with aspects of inventive concepts.

[0068] FIG. 3B shows an example graph of modeled transmission data for electromagnetic radiation filtered through a coarse wavelength reference 110 as a function of wavelength, in accordance with aspects of inventive concepts.

[0069] FIG. 4A shows a side view of an example embodiment of electromagnetic signals from the first and second fine wavelength beam directors filtering through the fine wavelength reference, in accordance with aspects of inventive concepts.

[0070] FIG. 4B shows an example of a modeled transmission intensity of the first fine wavelength detector plotted against a modeled transmission intensity of the second fine wavelength detector, in accordance with aspects of inventive concepts.

[0071] FIG. 4C shows an example of a modeled transmission intensity as a function of wavelength from the first and second fine wavelength detectors in a fine wavelength reference module, in accordance with aspects of inventive concepts.

[0072] FIG. 4D shows an example of signals from a swept source laser after processing the measurements from the quadrature sensing detectors (e.g., fine wavelength detectors).

[0073] FIG. 4E shows an example of the two processed signals plotted against each other.

[0074] FIG. 4F shows an example plot used for wavelength estimation.

[0075] FIG. 5 shows an example embodiment of a reference system, in accordance with aspects of inventive concepts.DETAILED DESCRIPTION

[0076] A description of example embodiments follows.

[0077] In various embodiments, the devices, systems, and methods described herein involve characterizing wavelength jitter between peaks using a coarse wavelength reference tuned to provide a monotonically increasing intensity spectrum between the wavelengths of interest.

[0078] In various example embodiments, the devices, systems, and methods described herein involve using a coarse wavelength reference and a fine wavelength reference to characterize wavelength jitter.

[0079] In various example embodiments, the devices, systems, and methods described herein involve using a coarse wavelength reference and calibration module to calibrate a coarse wavelength reference to identify drift.

[0080] A beam director as mentioned herein comprises any device suitable for redirecting at least a portion of incident electromagnetic radiation. Examples of beam directors may include, but are not limited to one or more beam splitters, one or more mirrors, one or more partial reflectors, one or more filters, and / or combinations thereof.

[0081] An etalon as mentioned herein comprises an optical cavity that includes at least two reflecting surfaces. In various embodiments, the reflecting surfaces are faces of a single piece of glass. In alternative embodiments, the etalon is configured differently. Coherent electromagnetic radiation filtering through a piece of glass with parallel surfaces will produce varying transmission as the wavelength is varied. Peak locations in the etalon spectrum are well defined and can be controlled by at least the thickness of the piece of glass, glass type, and temperature. This can result in a stable reference because the peaks will not shift much with small temperature changes and temperature can be well controlled.

[0082] A detector as mentioned herein comprises any device suitable for measuring an electromagnetic signal. Examples of detectors include, but are not limited to one or more photodiodes, one or more avalanche photodiode, etc. In the embodiments shown and described herein various detectors are assigned different element labels. For example, the detectors associated with the coarse wavelength reference 110 is labelled 106, the detector associated with the intensity reference module 200 is labelled 206, the detectors associated with the fine wavelength reference module 300 are labelled 306, and the detectors associated with the calibration module 400 are labelled 406. In various embodiments, there are different types of detectors. In various embodiments, one or more of these different detectors may be the same type of detector.

[0083] In the embodiments shown and described herein, the terms “electromagnetic radiation” and “electromagnetic signal” may be used interchangeably. In the embodiments shown straight lines and arrows are used to indicate electromagnetic radiation. In various embodiments, such radiation comprises a continuous wave of electromagnetic radiation. In various embodiments, such radiation comprises pulsed electromagnetic radiation. In various embodiments, such radiation comprises a stream of discrete photons. In various embodiments the electromagnetic radiation comprises a combination of a continuous wave, pulsed radiation, and discrete photons.

[0084] FIG. 1 shows an example embodiment of a reference system 500, in accordance with aspects of inventive concepts. In various embodiments, such as the one shown in FIG. 1,the reference system 500 comprises a wavelength reference module 100. In various embodiments, the wavelength reference module 100 comprises at least one coarse wavelength reference 110 configured to receive and filter an electromagnetic signal. In various embodiments, the wavelength reference module 100 comprises at least one coarse wavelength detector 106 configured to measure the intensity of at least a portion of the filtered electromagnetic signal. In the embodiment shown in FIG. 1 the reference system 500 comprises one wavelength reference module 100. In alternative embodiments, the reference system 500 comprises more than one wavelength reference module 100.

[0085] In various embodiments, such as the one shown in FIG. 1, the reference system 500 comprises an electromagnetic source 10 configured to output an electromagnetic signal. In various embodiments, the electromagnetic signal comprises optical wavelengths. In various embodiments, the electromagnetic signal comprises infrared wavelengths. In various embodiments, the electromagnetic signal comprises ultraviolet wavelengths. In various embodiments, the electromagnetic source 10 comprises a laser. In various embodiments, the electromagnetic source 10 comprises a tunable laser. In various embodiments, the electromagnetic source 10 comprises a broadband source.

[0086] In various embodiments, such as the one shown in FIG. 1, the electromagnetic source 10 comprises a collimator 12. In various embodiments, the electromagnetic source 10 does not comprise a collimator.

[0087] In various embodiments, the reference system 500 comprises at least one input polarizer 14 positioned between the electromagnetic source 10 and the wavelength reference module 100. In the embodiment shown in FIG. 1, the reference system 500 comprises one input polarizer 14. In alternative embodiments, the reference system 500 comprises more than one input polarizer 14. In alternative embodiments, the reference system 500 does not comprise an input polarizer 14.

[0088] In various embodiments, the wavelength reference module 100 comprises at least one coarse wavelength beam director 102 configured to receive an electromagnetic signal from the electromagnetic source 10 and redirect a portion of the electromagnetic signal towards the at least one coarse wavelength reference 110. In the embodiment shown in FIG. 1, the reference system 500 comprises one coarse wavelength beam director 102. In alternative embodiments, the reference system 500 comprises a different number of coarse wavelength beam directors 102.

[0089] In the embodiment shown in FIG. 1, the coarse wavelength beam director 102 directs a portion of the electromagnetic signal in a direction approximately orthogonal to the incident direction. In alternative embodiments, the coarse wavelength beam director 102 may direct a portion of the electromagnetic signal in a direction that is not orthogonal to the incident direction. In various embodiments, at least one input polarizer 14 comprises an extinction ratio high enough to suppress fluctuations from the coarse wavelength beam director 102 due to polarization changes.

[0090] In various embodiments, such as the one shown in FIG. 1, the at least one coarse wavelength reference 110 is positioned between the coarse wavelength beam director 102 and the coarse wavelength detector 106.

[0091] In various embodiments, the at least one coarse wavelength reference 110 comprises an etalon. In various embodiments, at least one of the at least one coarse wavelength reference 110 comprises an etalon.

[0092] FIG. 2A shows electromagnetic radiation filtering through an embodiment of an etalon and towards a detector, in accordance with aspects of inventive concepts. An etalon comprises an optical cavity that includes at least two reflecting surfaces. In various embodiments, the reflecting surfaces are faces of a single piece of glass. In alternative embodiments, the etalon is configured differently. Coherent electromagnetic radiation filtering through a piece of glass with parallel surfaces will produce varying transmission characteristics as the wavelength is varied (see FIG. 2B).

[0093] FIG. 2B is an example graph of modeled transmission intensity as a function of wavelength at a fixed position for electromagnetic radiation filtering through an etalon comprising 20 microns of BK7 glass. Peak locations in the etalon spectrum are well defined and can be controlled by at least the thickness of the piece of glass, glass type, and temperature. This can result in a stable reference because the peaks will not shift much with small temperature changes and temperature can be well controlled.

[0094] FIG. 3 A shows an example embodiment of a coarse wavelength reference 110, in accordance with aspects of inventive concepts. In various embodiments, such as the one shown in FIG. 3 A, the coarse wavelength reference 110 comprises more than one layer. In alternative embodiments, the coarse wavelength reference 110 comprises one layer. In various embodiments, such as the one shown in FIG. 3 A, the coarse wavelength reference 110 comprises four layers. In alternative embodiments, the coarse wavelength reference 110comprises a different number of layers. In embodiments that include more than one coarse wavelength reference 110, different coarse wavelength references 110 may comprise a different number of layers. In different embodiments the thickness of the coarse wavelength reference 110 may vary. In different embodiments, the thickness of each layer of the at least one coarse wavelength reference 110 may vary.

[0095] In various embodiments, such as the one shown in FIG. 3 A, at least one of the at least one coarse wavelength reference 110 comprises a first (bottom) layer 111, a second layer 112, a third layer 113, and a fourth (top) layer 115. In various embodiments, each of the layers comprises a different material. In various embodiments, one or more of the layers of at least one of the at least one coarse wavelength reference 110 comprises the same material.

[0096] In various embodiments, the layers of at least one of the at least one coarse wavelength reference 110 comprise the same thickness. In various embodiments, one or more of the one or more layers of at least one of the at least one coarse wavelength reference 100 comprises the same thickness. In various embodiments, each layer of at least one of the at least one coarse wavelength reference 100 comprises a different thickness. In various embodiments, the thickness of the coarse wavelength reference 110 is chosen based on the characteristics of the electromagnetic signal. For example, electromagnetic sources that are tunable over a broad wavelength range may need relatively thinner etalons to be resolved in a wavelength range of interest.

[0097] In various embodiments, such as the one shown in FIG. 3 A, one layer is substantially thicker than the other layers. In various embodiments, such as the one shown in FIG. 3A, the fourth layer 114 is substantially thicker than the other layers 111-113.

[0098] In various embodiments, such as the one shown in FIG. 3 A, the first layer 111 and the third layer 113 comprise the same material. In alternative embodiments, the different layers may comprise the same material. In alternative embodiments, each layer comprises a different material. In various embodiments, such as the one shown in FIG. 3A, the first layer 111 comprises silicon. In various embodiments, such as the one shown in FIG. 3 A, the second layer 112 comprises silicon dioxide. In various embodiments, such as the one shown in FIG. 3 A, the third layer 113 comprises silicon. In various embodiments, such as the one shown in FIG. 3 A, the fourth layer 114 comprises sapphire.

[0099] In various embodiments, at least one of the at least one coarse wavelength reference comprises two layers of silicon separated by a very thin spacer to form a stableetalon. In various embodiments, the spacer comprises air. In various embodiments, the very thin spacer has a thickness of about 1 micron.

[0100] In various embodiments, at least one of the at least one coarse wavelength reference 110 comprises at least one angled surface. In various embodiments, at least one of the at least one coarse wavelength reference 110 does not comprises an angled surface. In various embodiments, such as the one shown in FIG. 3 A, at least one of the at least one coarse wavelength reference 110 comprises one angled surface. In alternative embodiments, at least one of the at least one coarse wavelength reference 110 comprises a different number of angled surfaces. In various embodiments, such as the one shown in FIG. 3 A the fourth layer 114 of at least one of the at least one coarse wavelength reference 110 comprises an angled surface. In alternative embodiments, a different layer of at least one of the at least one coarse wavelength reference 110 may comprise an angled surface.

[0101] In various embodiments one or more layers of at least one of the at least one coarse wavelength reference 110 comprises one or more coatings. In various embodiments one or more layers of at least one of the at least one coarse wavelength reference 110 comprises one or more reflective coatings. In various embodiments, a sapphire layer comprises a reflective coating. In various embodiments, a silicon dioxide layer comprises a reflective coating.

[0102] In various embodiments, such as the one shown in FIG. 3 A at least one of the at least one coarse wavelength reference 110 is configured to be relatively resistant to temperature fluctuations. In various embodiments, at least one of the at least one coarse wavelength reference 110 is configured to resolve wavelength features on the scale of the wavelength range of interest.

[0103] In various embodiments, the at least one of the at least one coarse wavelength reference 110 is positioned at an optical bench. In various embodiments, at least one of the at least one coarse wavelength detector 106 is positioned at an optical bench. In various embodiments, at least one of the at least one coarse wavelength beam director is positioned at an optical bench.

[0104] FIG. 3B shows an example graph of modeled transmission data for electromagnetic radiation filtered through a coarse wavelength reference 110 as a function of wavelength, in accordance with aspects of inventive concepts. In various embodiments, the coarse wavelength reference 110 is constructed and arranged such that the transmissionintensity is monotonically increasing within the wavelength range of interest. In various embodiments, such as the one shown in FIG. 3B, the transmitted intensity is monotonically increasing over a wavelength range of 2100 nm to 2400 nm so that each wavelength has a uniquely associated intensity. In alternative embodiments, at least one of the at least one coarse wavelength reference 110 is configured so that the transmitted intensity monotonically increases over a different wavelength range. In some embodiments, at least one of the at least one coarse wavelength reference 110 is configured so that the transmitted intensity curve differs from the one shown in FIG. 3B.

[0105] In various embodiments, at least one of the at least one coarse wavelength reference comprises an absorptive element. In various embodiments, at least one of the at least one coarse wavelength reference comprises an absorptive element with a monotonic response to changes in the wavelength of the electromagnetic signal. In some embodiments, such an absorptive element comprises crystal. In some embodiments, such an absorptive element comprises glass.

[0106] In various embodiments, at least one of the at least one coarse wavelength reference 110 requires calibration to ensure that it is constructed and arranged such that the transmission intensity is monotonically increasing within the wavelength range of interest. In various embodiments, the electromagnetic signal intensity is measured (see following discussion of intensity reference module) and the intensity measured at the at least one coarse wavelength reference detector is normalized accordingly. In various embodiments, power variations are normalized out by simultaneously measuring the absolute power of the beam (for example using the intensity reference module 200).

[0107] Referring back to FIG. 1, in various embodiments, such as the one shown in FIG. 1, the reference system 500 comprises an intensity reference module 200 configured to measure the intensity of the electromagnetic signal. In various embodiments, such as the one shown in FIG. 1, the intensity reference module 200 comprises at least one intensity reference module detector 206 configured to measure the intensity of at least a portion of the electromagnetic signal and at least one intensity reference beam director 202 configured to redirect the at least a portion of the electromagnetic signal towards the at least one intensity reference module detector 206 and pass a portion of the electromagnetic signal to the sample.

[0108] In the embodiment shown in FIG. 1, the intensity reference beam director 102 directs a portion of the electromagnetic signal in a direction approximately orthogonal to theincident direction. In alternative embodiments, the intensity reference beam director 102 may direct a portion of the electromagnetic signal in a direction that is not orthogonal to the incident direction.

[0109] In the embodiment shown in FIG. 1, a portion of the electromagnetic signal transmits through the at least one coarse reference beam director 102 and propagates to the intensity reference module 200. In alternative embodiments, the arrangement of the wavelength reference module 100 and the intensity reference module 200 may be different and the at least one coarse wavelength beam director 102 is configured to receive an electromagnetic signal from an electromagnetic source and transmit a portion of the electromagnetic signal towards the at least one coarse wavelength reference 110. In such alternative embodiments, the at least one coarse wavelength beam director 102 may be configured to redirect, rather than transmit, a portion of the electromagnetic signal to the intensity reference module 200.

[0110] In various embodiments, the at least one coarse wavelength beam director 102 is configured to alternate between directing the electromagnetic signal to the wavelength reference module 100 and directing the electromagnetic signal to the intensity reference module 200.

[0111] In various embodiments, the intensity reference module 200 is positioned in front of the wavelength reference module 100. In such embodiments, the at least one input polarizer 14 is positioned between the electromagnetic source 10 and the intensity reference module 200.

[0112] In various embodiments, the reference system 500 comprises a wavelength reference module 100 and does not comprise an intensity reference module 200. In various embodiments, the reference system 500 comprises more than one intensity reference module 200.

[0113] Referring back to FIG. 1, in various embodiments, such as the one shown in FIG. 1, the reference system 500 comprises a fine wavelength reference module 300. In various embodiments, the fine wavelength reference module 300 comprises at least one fine wavelength reference beam director 302; at least one fine wavelength reference 310 configured to receive a first portion of the electromagnetic signal at a first incident angle and second portion of the electromagnetic signal at a second incident angle, the fine wavelength reference 310 being configured to filter the first portion of the electromagnetic signal and thesecond portion of the electromagnetic signal; a first fine wavelength detector 306a configured to measure the intensity of at least a portion of the filtered first portion of the electromagnetic signal; and a second fine wavelength detector 306b configured to measure the intensity of at least a portion of the filtered second portion of the electromagnetic signal.

[0114] In various embodiments, such as the one shown in FIG. 1, the fine wavelength reference module 300 is positioned next to at least one of the at least one coarse wavelength reference 110. In alternative embodiments, the fine wavelength reference module 300 may be in a different position.

[0115] In various embodiments, the wavelength reference module 100 comprises an intermediate beam director 104 configured to steer at least a portion of the electromagnetic signal to the fine wavelength reference module 300. In the embodiment shown, an intermediate beam director 104 is positioned between at least one of the at least one coarse wavelength beam director 102 and at least one of the at least one coarse wavelength reference 110. In alternative embodiments, the intermediate beam director may be arranged differently.

[0116] In various embodiments, such as the one shown in FIG. 1, the fine wavelength reference module 300 comprises a first fine wavelength beam director 302a configured to direct a first portion of the electromagnetic signal to the fine wavelength reference 310 at a first incident angle. In various embodiments, the fine wavelength reference module 300 comprises a second fine wavelength beam director 302b configured to direct a second portion of the electromagnetic signal to the fine wavelength reference 310 at a second incident angle.

[0117] In various embodiments, such as the one shown in FIG. 1, a portion of the electromagnetic signal from the first fine wavelength beam director 302a that filters through the fine wavelength reference 310 is collected at a first fine wavelength detector 306a. In various embodiments, such as the one shown in FIG. 1, a portion of the electromagnetic signal from the second fine wavelength beam director 302b that filters through the fine wavelength reference 310 is collected at a second fine wavelength detector 306b.

[0118] In alternative embodiments, the fine wavelength module 300 comprises more than two fine wavelength beam directors 302, each with a corresponding fine wavelength detector 306. In various embodiments, one or more of the fine wavelength beam directors 302 directs a portion of the electromagnetic signal to the fine wavelength reference at a different incident angle.

[0119] In various embodiments, the fine wavelength reference module 300 may comprise a fine wavelength beam director that is configured to rotate so that it can direct a portion of the electromagnetic signal towards the fine wavelength reference 310 at different incident angles.

[0120] FIG. 4A shows a side view of an embodiment of electromagnetic signals from the first and second fine wavelength beam directors 302a, 302b filtering through the fine wavelength reference 310, in accordance with aspects of inventive concepts herein.

[0121] In different embodiments the thickness of the fine wavelength reference 310 may vary. In various embodiments, the fine wavelength reference 310 may comprise more than one layer. In different embodiments the thickness of each layer of the fine wavelength reference 310 may vary. In various embodiments, such as the one in FIG. 1 and 4A, the fine wavelength reference 310 comprises an etalon that has a thickness t of about 100 microns. In various embodiments, the fine wavelength reference 310 comprises a single layer glass. In various embodiments, the fine wavelength reference 310 comprises characteristics described in connection with the coarse wavelength reference 110. In various embodiments, the thickness of the fine wavelength reference 310 is chosen based on the characteristics of the electromagnetic signal. For example, broad sources may need thinner etalons to be resolved between the wavelengths of interest.

[0122] FIG. 4A shows the portion of the electromagnetic signal directed by the first fine wavelength beam director 302 filtering through the fine wavelength reference 310 towards the first fine wavelength reference detector 306a. In various embodiments, such as this one, the electromagnetic signal directed by the first fine reference beam director 302a is incident at the fine wavelength reference 310 at an incident angle that is approximately orthogonal with the surface of the fine wavelength reference 310. In alternative embodiments, the electromagnetic signal directed by the first fine reference beam director 302a may be incident at the fine wavelength reference 310 at a different angle.

[0123] In various embodiments, such as this one, the electromagnetic signal directed by the second fine reference beam director 302b is incident at the fine wavelength reference 310 at an incident angle that is offset from orthogonal with the surface of the fine wavelength reference 310 by an angle 0. In alternative embodiments, the electromagnetic signal directed by the second fine reference beam director 302b may be incident at the fine wavelength reference 310 at a different angle.

[0124] The angular difference 9 between the incident angle from the first fine reference beam director 302a and the second fine wavelength beam director 302b is critical and creates stability because each beam uses the same fine wavelength reference 310.

[0125] FIG. 4B shows an example of a modeled transmission intensity of the first fine wavelength detector plotted against a modeled transmission intensity of the second fine wavelength detector, in accordance with aspects of inventive concepts.

[0126] FIG. 4C shows an example of a modeled transmission intensity as a function of wavelength from the first 306a and second 306b fine wavelength detectors in a fine wavelength reference module 300, in accordance with aspects of inventive concepts. An arrow shows how information presented in FIG. 4C corresponds to information presented in FIG. 4B.

[0127] When measured the transmitted intensity detected at the first 306a and second 306b fine wavelength detector are measured simultaneously, a relative phase can be determined. This relative phase has a mapping to wavelength.

[0128] In some embodiments, by modulating the wavelength and using a lock-in amplifier, a derivative of the intensity measured at the first 306a and second 306b fine wavelength detectors can be determined - making the system less sensitive to power changes.

[0129] The coarse wavelength reference helps to distinguish global phase of a detected signal. The quadrature setup discussed in connection with FIGS. 4A-4B provides fine resolution by increasing the derivative of transmission as a function of wavelength. The fine wavelength reference module 300 together with the coarse wavelength reference 110 can provide an absolute wavelength reference.

[0130] FIG. 4D shows an example of signals from a swept source laser after processing the measurements from the quadrature sensing detectors (e.g., fine wavelength detectors). One detector collects light from the beam at normal incidence and the other detects light from the beam with tilted incidence. The angle of the tilt is chosen in such a way that the etalon signal is shifted by approximately 90 degrees.

[0131] FIG. 4E shows an example of the two processed signals plotted against each other. At each wavelength, the detectors will produce a point on this plot. Due to the phase shift between the two signals, a circle is revealed when the two are plotted against each other. This allows for the computation of the angle 0, which is functionally related to the actual wavelength.

[0132] FIG. 4F shows an example plot used for wavelength estimation. After computation of the angle 9, it can easily be converted to wavelength if the functional form has been previously established by calibration. Inset in the plot is a zoom in of the laser sweep. The laser used for this demonstration is particularly noisy and helps to illustrate the wavelength measurement capabilities of this method.

[0133] FIG. 5 shows an example embodiment of a reference system 500, in accordance with aspects of inventive concepts. In various embodiments, such as the one shown in FIG. 5, the reference system 500 comprises a calibration module 400. In various embodiments, the calibration module 400 comprises at least one calibration module beam director 402; at least one calibration reference 410 configured to receive a portion of the electromagnetic signal, the calibration reference 410 being configured to filter the portion of the electromagnetic signal; and a calibration detector 406 configured to measure the intensity of at least a portion of the filtered electromagnetic signal.

[0134] In various embodiments, such as the one shown in FIG. 5, the calibration module 400 is positioned next to at least one of the at least one coarse wavelength reference 110. In alternative embodiments, the calibration module 400 may be in a different position.

[0135] In various embodiments, the wavelength reference module 100 comprises an intermediate beam director 104 configured to steer at least a portion of the electromagnetic signal to the calibration module 400. In the embodiment shown, an intermediate beam director 104 is positioned between at least one of the at least one coarse wavelength beam director 102 and at least one of the at least one coarse wavelength reference 110. In alternative embodiments, the intermediate beam director may be arranged differently.

[0136] In various embodiments, such as the one shown in FIG. 5, the calibration module 400 comprises a calibration beam director 402 configured to direct a portion of the electromagnetic signal to the calibration reference 410. In various embodiments, the calibration module 400 may comprise a second calibration beam director configured to direct a second portion of the electromagnetic signal to the calibration reference 410.

[0137] In various embodiments, such as the one shown in FIG. 5, a portion of the electromagnetic signal from the calibration beam director 402 that filters through the calibration reference 410 is collected at a calibration detector 406.

[0138] In alternative embodiments, the calibration module 400 comprises more than one calibration beam directors 402, each with a corresponding calibration detector 406. In variousembodiments, one or more of the calibration beam directors 402 directs a portion of the electromagnetic signal to the calibration reference at a different incident angle.

[0139] In various embodiments, the calibration module 400 may comprise a calibration beam director that is configured to rotate so that it can direct a portion of the electromagnetic signal towards the calibration reference 410 at different incident angles.

[0140] In different embodiments the thickness of the calibration reference 410 may vary. In various embodiments, the calibration reference may comprise more than one layer. In different embodiments the thickness of each layer of the calibration reference 410 may vary. In various embodiments, such as the one in FIG. 5, the calibration reference comprises an etalon that has a thickness of about 100 microns. In various embodiments, the calibration reference comprises silicon. In various embodiments, the calibration reference 410 comprises a single layer glass. In various embodiments, the calibration reference 410 comprises characteristics described in connection with the coarse wavelength reference 110. In various embodiments, the thickness of the calibration reference 310 is chosen based on the characteristics of the electromagnetic signal. For example, broad sources will need thinner etalons to be resolved between the wavelengths of interest.

[0141] In various embodiments, the change in the etalon peak location is 0.12 nm / C for silicon. In various embodiments, the change in the etalon peak location is 0.025 nm / C for sapphire. In various embodiments, the change in the etalon peak location is 0.015 nm / C for silicon dioxide. In various embodiments, sapphire layers and / or silicon dioxide layers comprise a reflective coating.

[0142] In various embodiments, transmission intensity peaks that are detected as the electromagnetic signal filters through in the calibration reference 410 and the wavelength of the electromagnetic signal is swept through a range. In various embodiments, these transmission intensity peaks can be used as a reference to calibrate transmission intensity peaks that are detected as the electromagnetic signal filters through the coarse wavelength reference 110 and the wavelength of the electromagnetic signal is swept through a range.

[0143] In various embodiments the wavelength of the electromagnetic signal is swept through a range of interest. Transmission intensity peaks (as a function of wavelength) for the calibration reference 410 and the coarse wavelength reference 110 are compared. In such embodiments, the peaks from the calibration reference 410 are stable and may be used to “calibrate” the coarse wavelength reference. In various embodiments, the calibrationreference comprises a fused silica etalon that is lOOum thick and its transmission peak at 2300 nm will drift only by 0.015nm / C. In various embodiments, the information from the coarse wavelength reference (normalized to power) is then mapped to wavelength fluctuations.

[0144] In various embodiments, the calibration reference module 400 is used with electromagnetic signals at a fixed wavelength. In such embodiments, a user may sweep the electromagnetic signal through a range of wavelengths, find the transmission peaks, and calibrate the coarse wavelength reference. In various embodiments, a user may then select a specific wavelength for the electromagnetic signal and measure the transmitted intensity that filters through the coarse wavelength reference 110. There should not be significant drift between the data collected with the range of wavelengths and the data collect from a specific wavelength because most drift associated with the data from the coarse wavelength reference 110 would be related to temperature.

[0145] The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

[0146] The various features described herein can be used separately and / or in combination.

[0147] While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A reference system, comprising: a wavelength reference module, comprising: at least one coarse wavelength reference configured to receive an electromagnetic signal, the coarse wavelength reference being configured to filter the electromagnetic signal; and at least one coarse wavelength detector configured to measure the intensity of at least a portion of the filtered electromagnetic signal.

2. The reference system of claim 1, or any one of the preceding claims, wherein the electromagnetic signal comprises optical wavelengths.

3. The reference system of claim 1, or any one of the preceding claims, wherein the electromagnetic signal comprises infrared wavelengths.

4. The reference system of claim 1, or any one of the preceding claims, wherein the electromagnetic signal comprises ultraviolet wavelengths.

5. The reference system of claim 1, or any one of the preceding claims, wherein the electromagnetic signal is generated by a laser.

6. The reference system of claim 1, or any one of the preceding claims, wherein the electromagnetic signal is generated by a tunable laser.

7. The reference system of claim 1, or any one of the preceding claims, wherein the wavelength reference module comprises a coarse wavelength beam director configured to receive an electromagnetic signal from an electromagnetic source and redirect a portion of the electromagnetic signal towards the at least one coarse wavelength reference.

8. The reference system of claim 1, or any one of the preceding claims, wherein the wavelength reference module comprises a coarse wavelength beam director configured to receive an electromagnetic signal from an electromagnetic source and transmit a portion of the electromagnetic signal towards the at least one coarse wavelength reference.

9. The reference system of claim 7 or 8, or any one of the preceding claims, wherein the at least one coarse wavelength reference is positioned between the coarse wavelength beam director and the coarse wavelength detector.

10. The reference system of claim 7, or any one of the preceding claims, wherein the coarse wavelength beam director directs a portion of the electromagnetic signal in a direction approximately orthogonal to the incident direction.

11. The reference system of claim 8, or any one of the preceding claims, wherein the coarse wavelength beam director comprises a beam splitter.

12. The reference system of claim 8, or any one of the preceding claims, wherein the coarse wavelength beam director comprises a dichroic filter.

13. The reference system of claim 8, or any one of the preceding claims, wherein the coarse wavelength beam director is configured to rotate.

14. The reference system of claim 8, or any one of the preceding claims, wherein the coarse wavelength beam director comprises a mirror.

15. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises an etalon.

16. The reference system of claim 1, or any one of the preceding claims, wherein the coarse wavelength reference comprises more than one layer.

17. The reference system of claim 16, or any one of the preceding claims, wherein each layer of the at least one coarse wavelength reference comprises a different material.

18. The reference system of claim 16, or any one of the preceding claims, wherein one or more layers of at least one of the at least one coarse wavelength reference comprises the same material.

19. The reference system of claim 1, or any one of the preceding claims, wherein the coarse wavelength reference comprises four layers.

20. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises a layer of sapphire, a layer of silicon dioxide, and two layers of silicon.

21. The reference system of claim 16, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises a layer of sapphire directly above a first layer of silicon, the first layer of silicon being directly able a layer of silicon dioxide, the layer of silicon dioxide being directly above a second layer of silicon.

22. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises a first (bottom) layer, a second layer, a third layer, and a fourth (top) layer.

23. The reference system of claim 22, or any one of the preceding claims, wherein the first layer and the third layer comprise the same material.

24. The reference system of claim 22, or any one of the preceding claims, wherein the first layer comprises silicon.

25. The reference system of claim 22, or any one of the preceding claims, wherein the second layer comprises silicon dioxide.

26. The reference system of claim 22, or any one of the preceding claims, wherein the third layer comprises silicon.

27. The reference system of claim 22, or any one of the preceding claims, wherein the fourth layer comprises sapphire.

28. The reference system of claim 16, or any one of the preceding claims, wherein each layer of at least one of the at least one coarse wavelength reference comprises a different thickness.

29. The reference system of claim 16, or any one of the preceding claims, wherein each layer of at least one of the at least one coarse wavelength reference comprises the same thickness.

30. The reference system of claim 16, or any one of the preceding claims, wherein one or more layers of at least one of the at least one coarse wavelength reference comprises the same thickness.

31. The reference system of claim 22, or any one of the preceding claims, wherein the fourth layer is thicker than the other layers.

32. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises at least one angled surface.

33. The reference system of claim 22, or any one of the preceding claims, wherein the fourth layer of at least one of the at least one coarse wavelength reference comprises an angled surface.

34. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises a coating.

35. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference comprises a reflective coating.

36. The reference system of claim 16, or any one of the preceding claims, wherein one or more layers of at least one of the at least one coarse wavelength reference comprises one or more coatings.

37. The reference system of claim 16, or any one of the preceding claims, wherein one or more layers of at least one of the at least one coarse wavelength reference comprises one or more reflective coatings.

38. The reference system of claim 21, or any one of the preceding claims, wherein a sapphire layer comprises a reflective coating.

39. The reference system of claim 21, or any one of the preceding claims, wherein a silicon dioxide layer comprises a reflective coating.

40. The reference system of claim 1, or any one of the preceding claims, wherein the intensity measured at the at least one coarse wavelength reference detector is normalized to the electromagnetic signal intensity.

41. The reference system of claim 1, or any one of the preceding claims, further comprising an input polarizer constructed and arranged to polarize the electromagnetic signal that is received by the at least one coarse wavelength reference.

42. The reference system of claim 1, or any one of the preceding claims, wherein the wavelength of the electromagnetic signal is swept through a range of 2100 nm to 2400 nm.

43. The reference system of claim 1, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference is configured such that a transmitted intensity is monotonically increasing over a wavelength range of 2100 nm to 2400 nm.

44. The reference system of claim 1, or any one of the preceding claims, further comprising an intensity reference module, the intensity reference module comprising: at least one intensity reference module detector configured to measure the intensity of at least a portion of the electromagnetic signal; and at least one intensity reference beam director configured to redirect the at least a portion of the electromagnetic signal towards the at least oneintensity reference module detector and pass a portion of the electromagnetic signal to the sample.

45. The reference system of claim 44, or any one of the preceding claims, wherein the intensity reference beam director directs a portion of the electromagnetic signal in a direction approximately orthogonal to the incident direction.

46. The reference system of claim 44, or any one of the preceding claims, wherein at least a portion of the electromagnetic signal transmits through the at least one coarse reference beam director and propagates to the intensity reference module.

47. The reference system of claim 44, or any one of the preceding claims, wherein the at least one coarse wavelength beam director may be configured to redirect, rather than transmit, a portion of the electromagnetic signal to the intensity reference module.

48. The reference system of claim 44, or any one of the preceding claims, wherein the at least one coarse wavelength beam director is configured to alternate between directing the electromagnetic signal to the wavelength reference module and directing the electromagnetic signal to the intensity reference module.

49. The reference system of claim 44, or any one of the preceding claims, wherein the at least one input polarizer is positioned between the electromagnetic source and the intensity reference module.

50. The reference system of claim 1, or any one of the preceding claims, further comprising a fine wavelength reference module, the fine wavelength reference module comprising: at least one fine wavelength beam director; at least one fine wavelength reference configured to receive a first portion of the electromagnetic signal at a first incident angle and second portion of the electromagnetic signal at a second incident angle, the fine wavelength reference being configured to filter the first portion of the electromagnetic signal and the second portion of the electromagnetic signal;a first fine wavelength detector configured to measure the intensity of at least a portion of the filtered first portion of the electromagnetic signal; and a second fine wavelength detector configured to measure the intensity of at least a portion of the filtered second portion of the electromagnetic signal.

51. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength beam director comprises a beam splitter.

52. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength beam director comprises a dichroic filter.

53. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength beam director is configured to rotate.

54. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength beam director comprises a mirror.

55. The reference system of claim 50, or any one of the preceding claims, wherein the at least one fine wavelength beam director comprises a first fine wavelength beam director configured to direct the first portion of the electromagnetic signal to the at least one fine wavelength reference and a second fine wavelength beam director configured to direct the second portion of the electromagnetic signal to the at least one fine wavelength reference.

56. The reference system of claim 50, or any one of the preceding claims, wherein the wavelength reference module further comprises an intermediate beam director configured to steer at least a portion of the electromagnetic signal to the fine wavelength reference module.

57. The reference system of claim 56, or any one of the preceding claims, wherein the intermediate beam director is positioned between at least one of the at least one coarsewavelength beam director and at least one of the at least one coarse wavelength reference.

58. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength reference comprises an etalon.

59. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength reference includes an etalon comprising a thickness of 100 microns.

60. The reference system of claim 50, or any one of the preceding claims, wherein the fine wavelength reference comprises a single layer glass.

61. The reference system of claim 1, or any one of the preceding claims, further comprising a calibration module comprising: at least one calibration reference configured to receive a portion of the electromagnetic signal, the calibration reference being configured to filter the first portion of the electromagnetic signal and the second portion of the electromagnetic signal; at least one calibration beam director configured to direct a portion of the electromagnetic signal to the at least one calibration reference; and at least one calibration detector configured to measure the intensity of at least a portion of the filtered electromagnetic signal.

62. The reference system of claim 61, or any one of the preceding claims, wherein the wavelength reference module comprises an intermediate beam director configured to steer at least a portion of the electromagnetic signal to the calibration module.

63. The reference system of claim 61, or any one of the preceding claims, wherein the intermediate beam director is positioned between at least one of the at least one coarse wavelength beam director and at least one of the at least one coarse wavelength reference.

64. The reference system of claim 61, or any one of the preceding claims, wherein the calibration module comprises a first calibration beam director configured to direct a first portion of the electromagnetic signal to the calibration reference a second calibration beam director configured to direct a second portion of the electromagnetic signal to the calibration reference.

65. The reference system of claim 61, or any one of the preceding claims, wherein the calibration module comprises at least one calibration beam director that is configured to rotate so that it can direct a portion of the electromagnetic signal towards the calibration reference at different incident angles.

66. The reference system of claim 61, or any one of the preceding claims, wherein the calibration reference comprises an etalon.

67. The reference system of claim 61, or any one of the preceding claims, wherein the calibration reference includes an etalon comprising a thickness of 100 microns.

68. The reference system of claim 61, or any one of the preceding claims, wherein the calibration reference comprises more than one layer.

69. The reference system of claim 61, or any one of the preceding claims, wherein the calibration reference comprises silicon.

70. The reference system of claim 61, or any one of the preceding claims, wherein the calibration reference comprises a single layer of glass.

71. The reference system of claim 1, or any one of the preceding claims, wherein the at least one coarse wavelength reference is positioned at an optical bench.

72. The reference system of claim 1, or any one of the preceding claims, wherein the at least one coarse wavelength detector is positioned at an optical bench.

73. The reference system of claim 7 or 8, or any one of the preceding claims, wherein the at least one coarse wavelength beam director is positioned at an optical bench.

74. The reference system of claim 50, or any one of the preceding claims, wherein the at least one fine wavelength reference is positioned at an optical bench.

75. The reference system of claim 50, or any one of the preceding claims, wherein the at least one fine wavelength detector is positioned at an optical bench.

76. The reference system of claim 50, or any one of the preceding claims, wherein the at least one fine wavelength beam director is positioned at an optical bench.

77. The reference system of claim 61, or any one of the preceding claims, wherein the at least one calibration reference is positioned at an optical bench.

78. The reference system of claim 61, or any one of the preceding claims, wherein the at least one calibration detector is positioned at an optical bench.

79. The reference system of claim 61, or any one of the preceding claims, wherein the at least one calibration beam director is positioned at an optical bench.

80. The reference system of claim 56 or 62, or any one of the preceding claims, wherein the at least one intermediate beam director is positioned at an optical bench.

81. The reference system of claim 40, or any one of the preceding claims, wherein the at least one intensity reference module beam director is positioned at an optical bench.

82. The reference system of claim 44, or any one of the preceding claims, wherein the at least one intensity reference module detector is positioned at an optical bench.

83. A method of referencing an electromagnetic signal, comprising: providing a reference system, comprising: a wavelength reference module, comprising: at least one coarse wavelength reference configured to receive an electromagnetic signal; andat least one coarse wavelength detector; filtering the electromagnetic signal through the at least one coarse wavelength reference; and measuring the intensity of at least a portion of the electromagnetic signal that filters through the coarse wavelength reference at the at least one coarse wavelength detector.

84. The method of claim 83, or any one of the preceding claims, further comprising normalizing the intensity measured at the at least one coarse wavelength reference detector to the electromagnetic signal intensity.

85. The method of claim 83, or any one of the preceding claims, further comprising adjusting the wavelength of the electromagnetic signal between 2100 nm to 2400 nm.

86. The method of claim 83, or any one of the preceding claims, wherein at least one of the at least one coarse wavelength reference is configured such that the transmitted intensity is monotonically increasing over a wavelength range of 2100 nm to 2400 nm.

87. The method of claim 83, or any one of the preceding claims, further comprising: providing a fine wavelength reference module, the fine wavelength reference module comprising: at least one fine wavelength reference; a first fine wavelength detector; and a second fine wavelength detector filtering a first portion of the electromagnetic signal though at least one of the at least one fine wavelength reference at a first incident angle; filtering a second portion of the electromagnetic signal though the at least one of the at least one fine wavelength reference at a second incident angle;measuring the intensity of the first portion of the electromagnetic signal that filters through the fine wavelength reference at the first incident angle at the first fine wavelength detector; and measuring the intensity of the second portion of the electromagnetic signal that filters through the fine wavelength reference at the second incident angle at the second fine wavelength detector.

88. The method of claim 87, or any one of the preceding claims, further comprising: providing at least one fine wavelength beam director; and directing a portion of the electromagnetic signal to at least of the at least one fine wavelength beam director.

89. The method of claim 83, or any one of the preceding claims, further comprising: providing a calibration module comprising: at least one calibration reference; and at least one calibration detector; filtering a portion of the electromagnetic signal through the at least one calibration reference; and measuring an intensity of at least a portion of the electromagnetic signal that filters through the calibration reference at the at least one calibration detector.

90. The reference system of claim 1, or any one of the preceding claims, wherein the at least one coarse wavelength reference comprises an absorptive element with a monotonic response to changes in the wavelength of the electromagnetic signal.

91. The reference system of claim 90, or any one of the preceding claims, wherein the at least one coarse wavelength reference comprises crystal.

92. The reference system of claim 90, or any one of the preceding claims, wherein the at least one coarse wavelength reference comprises glass.

93. The reference system of claim 1, or any one of the preceding claims, wherein the at least one coarse wavelength reference comprises two layers of silicon separated by a spacer to form a stable etalon.

94. The reference system of claim 93, or any one of the preceding claims, wherein the spacer comprises air.

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