Frequency comb generation based on electro-optical phase-coded mode locking for cyclic ranging OCT

Abstract

A source for providing electromagnetic radiation within a specific spectral range, the source comprising: a ring optical resonator for circulating multiple bands, the ring optical resonator comprising: a first optical phase modulator, a first dispersive device, a second optical phase modulator, a multi-line spectral domain filter, a second dispersive device, and an optical amplifier; a controller coupled to the first optical phase modulator and the second optical phase modulator, the controller being configured to drive the first optical phase modulator with a first waveform and to drive the second optical phase modulator with a second waveform, the first dispersive device being configured between the first optical phase modulator and the second optical phase modulator to provide dispersion, thereby subjecting each of the multiple bands to a corresponding plurality of different time delays, the first optical phase modulator and the second optical phase modulator being configured to create spectral broadening.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based upon and claims priority from U.S. patent application serial number 62 / 968,299, filed on January 31, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0003] A Note About Federally Funded Research

[0004] This invention was made with Government support under P41EB015903 awarded by the National Institutes of Health and FA9550-11-1-0331 awarded by the Air Force Office of Scientific Research. The Government has certain rights in this invention. Background Art

[0005] Optical coherence tomography (OCT) is widely used to image the three-dimensional structure of biological tissues and materials. A defining characteristic of OCT technology is the use of echo delay ranging to resolve the depth position of scatterers. In the recently described cyclic ranging (CR) OCT method, compressed echo delay ranging is used to reduce the number of measurements required to capture the signal in the depth domain by exploiting sparsity. This sparsity is common in OCT, especially in long-range OCT applications. By reducing the measurement count, the electronic signal capture and processing bandwidth is reduced, enabling high-speed imaging for a given electronic bandwidth or a simplified signal capture system for moderate-speed imaging. To image at the highest possible speeds, stretched-pulse mode-locked (SPML) lasers provide the stepped frequency comb output used by CR-OCT with repetition rates ranging from several megahertz to tens of megahertz. These SPML sources are not easily scalable to slower speeds and therefore do not provide a solution for moderate-speed CR-OCT. Instead, a different source technology is required to achieve moderate-speed CR-OCT. Summary of the Invention

[0006] Accordingly, new systems, methods, and apparatus for providing a source for annular ranging OCT are needed.

[0007] In one embodiment, the present invention provides a source for providing electromagnetic radiation within a specific spectral range, comprising: a ring optical resonator, the ring optical resonator being configured to circulate a plurality of wavelength bands, the ring optical resonator comprising: a first optical phase modulator, a first dispersive device, a second optical phase modulator, a multi-line spectral domain filter, a second dispersive device, and an optical amplifier; a controller coupled to the first optical phase modulator and the second optical phase modulator, the controller being configured to drive the first optical phase modulator with a first waveform and to drive the second optical phase modulator with a second waveform, the first dispersive device being configured between the first optical phase modulator and the second optical phase modulator to provide dispersion, thereby subjecting each wavelength band in the plurality of wavelength bands to a corresponding plurality of different time delays, the first optical phase modulator and the second optical phase modulator being configured to generate spectral broadening for each wavelength band through the first optical phase modulator, and to generate spectral broadening for each wavelength band through the first optical phase modulator, and to generate spectral broadening for each wavelength band through the second optical phase modulator. A first phase is modulated using the first optical phase modulator driven by a first waveform to perform spectral recovery for a specific band among the multiple bands through a second optical phase modulator, and after a specific time delay, a second phase is modulated using a second optical phase modulator driven by a second waveform, the second waveform including an inverse of the first waveform, the specific time delay being determined to create spectral recovery for the specific band among the multiple bands, the multi-line spectral domain filter is configured to provide multi-line spectral filtering with a narrow bandwidth to induce power loss for each of the multiple bands except the specific band, the second dispersion device is configured to provide dispersion compensation to the output of the multi-line spectral domain filter to compensate for group delay dispersion within the optical resonator and match the round-trip frequency of each of the multiple bands, and the first waveform and the second waveform are configured to create periodic phase modulation to recover the multiple bands at a frequency that is an integer multiple of the round-trip frequency of the optical resonator.

[0008] In another embodiment, the present invention provides a source for providing electromagnetic radiation within a specific spectral range, comprising: a ring optical resonator for circulating a ring of multiple wavelength bands, the ring optical resonator comprising: a first optical phase modulator, a dispersive device, a second optical phase modulator, a multi-line spectral domain filter, and an optical amplifier; a controller coupled to the first optical phase modulator and the second optical phase modulator, the controller configured to drive the first optical phase modulator with a first waveform and drive the second optical phase modulator with a second waveform, the dispersive device being disposed between the first optical phase modulator and the second optical phase modulator to provide dispersion, thereby subjecting each of the multiple wavelength bands to a corresponding plurality of different time delays, the first optical phase modulator and the second optical phase modulator being configured to create spectral broadening for each of the multiple wavelength bands via the first optical phase modulator, and performing spectral restoration for a specific wavelength band among the plurality of wavelength bands by modulating a first phase using the first optical phase modulator driven by a first waveform, and after a specific time delay, modulating a second phase using the second optical phase modulator driven by a second waveform, the second waveform including an inverse of the first waveform, the specific time delay being determined so as to create spectral restoration for the specific wavelength band among the plurality of wavelength bands, the multiline spectral domain filter being configured to provide multiline spectral filtering having a narrow bandwidth so as to induce power loss for each wavelength band among the plurality of wavelength bands except the specific wavelength band, the first waveform and the second waveform being configured to create restoration having a sufficiently long duration so that each of the plurality of wavelength bands undergoes a plurality of round trips within the optical resonator, and the first waveform and the second waveform being configured to create periodic phase modulation to restore the plurality of wavelength bands at a frequency that is an integer multiple of a round trip frequency of the optical resonator. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Various objects, features, and advantages of the disclosed subject matter may be more fully understood by reference to the following detailed description of the disclosed subject matter when considered in conjunction with the following drawings, in which like reference numerals identify like elements.

[0010] Figure 1 Figures depicting the operating principles of phase-coded wavelength filters are shown. Panel (a) illustrates linewidth broadening and restoration of narrowband light through phase modulation and compensation. Panel (b) illustrates selective linewidth restoration using a frequency comb and wavelength filtering using a Fabry-Perot etalon with dispersion between two phase modulators. PM, phase modulator; FG, function generator; SOA, semiconductor optical amplifier.

[0011] Figure 2Figure 1 shows the PCML laser setup in panel (a), and the RF waveform design for the phase modulator in panel (b). AWG, arbitrary waveform generator; PM, phase modulator; PC, polarization controller; FPE, Fabry-Perot etalon; DCF, dispersion-compensating fiber; FRM, Faraday rotator mirror; SMF, single-mode fiber; SOA, semiconductor optical amplifier; ISO, isolator.

[0012] Figure 3 Single-pass phase-coded filter performance measurements for each comb line are shown. The linewidth (panels a, c, e, g) and intensity (panels b, d, f, h) are measured after passing through the first etalon (a, b); after linewidth stretching by the first modulator (c, d); after linewidth restoration by the second modulator (e, f); and after filtering by the second etalon (g, h).

[0013] Figure 4 The laser spectrum (left), the spectrum in the wavelength range indicated by the red box on the left (middle), and the time trace (right) of a PCML laser operated under the following conditions are shown: (inset a) 176 kHz, 130 wavelengths, t p =43ns; (Small Figure b) 881kHz, 130 wavelengths, t p =8.6ns; and (panel c) 3.52MHz, 31 wavelengths, t p =8.6ns.

[0014] Figure 5 6dB roll-off measurements of PCML lasers with various configurations are shown. Panel (a) shows 176kHz without a boosted SOA, panel (b) shows 176kHz with a boosted SOA, panel (c) shows 881kHz without a boosted SOA, and panel (d) shows 881kHz with a boosted SOA.

[0015] Figure 6 Cross-sectional images of an IR card (panels a-c) and a finger (panels d, e) with different A-line configurations at the same scale and corresponding cycle range are shown. Data were collected at three different imaging speeds: (a, d) 176 kHz, (b, e) 881 kHz, and (c) 3.52 MHz. The dynamic ranges were (a, b) 20 dB, (c) 10 dB, and (d, e) 18 dB. Scale bar (lower left corner of panel a): 200 μm.

[0016] Figure 7, inset (a) shows the PCML A-line architecture with laser on / off operation, and inset (b) shows the AWG waveform design of each modulator for laser on / off operation.

[0017] Figure 8 Panel (a) shows the orthogonal RIN for a single-pulse experiment, where the red / blue / green / purple dots correspond to 150 / 300 / 500 / 8000 finesse at 1560 nm, respectively. Panel (b) shows the orthogonal RIN plots for a double-pulse experiment with finesse of 150 / 300 / 500 / 8000, respectively. The blue dot corresponds to 1560 nm, which is the first position, and the orange dot corresponds to 1559 nm, which is the second position.

[0018] Figure 9 , Panel (a) shows the OSA spectrum of the PCML laser. (b) The time trace and orthogonal RIN at the peak of each pulse.

[0019] Figure 10 , inset (a) shows a non-averaged cross-sectional image of a finger acquired from the PCML-OCT system, while inset (b) shows a cross-sectional image of the finger averaging 25 out-of-plane images at the same position.

[0020] Figure 11 An example of a system for providing a source for cyclic ranging OCT is shown, in accordance with some embodiments of the disclosed subject matter.

[0021] Figure 12 Shown are examples of hardware that can be used to implement computing devices and servers according to some embodiments of the disclosed subject matter.

[0022] Figure 13 Figures are provided of interferometry systems that may be used in conjunction with various embodiments disclosed herein, with inset (A) showing a Mach-Zehnder type interferometer that may be implemented using free-space optics and inset (B) showing a fiber optic arrangement. DETAILED DESCRIPTION

[0023] According to some embodiments of the disclosed subject matter, mechanisms (which may include systems, methods, and devices) are provided for providing a source for cyclic ranging OCT, particularly at moderate speeds.

[0024] Cyclic ranging (CR) optical coherence tomography (OCT) uses frequency comb sources to improve long-range imaging. While the initial development of CR-OCT focused on ultrahigh-speed imaging (i.e., operating at A-line rates of several MHz to tens of MHz), there are many applications and imaging strategies where more modest speeds are preferred. However, a suitable frequency comb source is lacking to enable moderate-speed CR-OCT imaging. Described herein are embodiments of a novel phase-code mode-locked (PCML) laser architecture that can operate in the kilohertz to megahertz range while also providing novel features such as dynamic reconfiguration and simplified linear-in-time frequency stepping. A prototype CR-OCT system with a PCML laser is demonstrated, and imaging results are presented at A-line rates from 176 kHz to 3.52 MHz, with a coherence length-limited imaging depth of 170 mm. In various embodiments of the disclosed apparatus, A-line rates from 100 kHz to 5 MHz can be achieved. In further various embodiments of the disclosed apparatus, a sequence of light pulses at an optical frequency comb line may be achieved, each pulse having a pulse width of 1 ns to 100 ns.

[0025] Optical coherence tomography (OCT) is a widely used three-dimensional imaging modality defined by depth ranging using echo delay. In the recently described cyclic ranging (CR) OCT method, compressed echo delay ranging is achieved using a time-frequency stepped comb source. Using compressed ranging, fewer measurements are required to interrogate long depth ranges. This reduces the bandwidth requirements of the electronics used to capture and process the output signals. CR-OCT has been demonstrated at speeds exceeding 10 MHz using a high-speed stepped frequency comb source based on stretched pulse mode locking (SPML).

[0026] The compression provided by CR can also be used to reduce the electronic bandwidth requirements in moderate-speed tele-imaging applications. Unfortunately, SPML lasers are not suitable for moderate-speed imaging; to reduce the speed below a few megahertz, long-chirped fiber Bragg gratings need to be used. Instead, moderate-speed CR-OCT relies on a source created by modifying the existing swept-source laser architecture. A fixed Fabry-Perot etalon is added to a polygon mirror and a micromachined Fabry-Perot swept-wavelength laser. While this produces the desired stepped frequency comb output, the noise and coherence length characteristics are relatively poor, and the temporally nonlinear output pulse train complicates signal capture.

[0027] Accordingly, embodiments of a novel stepped frequency comb laser architecture for use in CR-OCT are disclosed herein. This laser architecture is known as phase-coded mode-locked (PCML). Like SPML lasers, PCML lasers use intracavity dispersion and electro-optical modulation rather than mechanically tuned spectral filters. The difference between the SPML architecture and the PCML architecture is that the SPML architecture uses electro-optical amplitude modulation, while the PCML architecture uses electro-optical phase modulation. This distinction, together with the use of appropriately designed control signals, allows the laser repetition rate to be decoupled from the intracavity dispersion. Thus, embodiments of the PCML laser disclosed herein are capable of operating at moderate speeds with reasonable levels of intracavity dispersion. Furthermore, without hardware modifications, the PCML source can operate over a wide range of speeds and can generate nearly arbitrary sequences of comb lines (i.e., time-sequential output wavelengths).

[0028] PCML lasers use reversible electro-optical line broadening to create frequency comb line transmission filters. This principle is relatively simple, such as Figure 1 As a starting point, we first analyze the evolution of the narrowband CW input field at the optical frequency ω and use E A (t)=cos(ωt) to describe ( Figure 1 (a)). Light is emitted at point A. The first modulator is driven by the voltage drive signal f1(t), generating B The output field is given by (t) = cos(ωt) + αf1(t), where α is the modulator proportionality factor (rad / V) between the induced phase shift and the drive voltage. For simplicity, α is assumed to be constant between RF and optical frequencies.

[0029] like Figure 1 As shown in (a), this phase modulation broadens the line width of the light at point B. The spectrally broadened light travels to the second phase modulator located at a distance d from the first modulator (in the fiber, point C). The field at this time is determined by E C (t) = cos(ω(t-dn / c)+αf1(t-dn / c)), where n is the group refractive index of the fiber and c is the speed of light. At the output of the second phase modulator (point D), the field is further modulated to E D It is easy to understand that if the driving signal is configured so that f2(t) = -f1(t-dn / c), the two modulations cancel and the original narrowband linewidth is restored, E D (t) = cos(ωt). Note that this reversible linewidth broadening occurs only when the delay of signal f2 relative to f1 matches the optical group delay between the modulators.

[0030] exist Figure 1In (b), a comb line transmission filter is constructed based on the principle of reversible linewidth broadening. Here, identical Fabry-Perot etalons are added before and after the phase modulator, and a dispersive fiber is added between the modulators. If broadband (e.g. amplified spontaneous emission (ASE)) light is emitted, the first etalon will produce a frequency comb. Each line of this frequency comb will be equivalently broadened by the first phase modulator. Now, due to the dispersive fiber, each of the optical comb lines experiences a different group delay when it is transmitted to the second modulator. The second modulator drive signal f2 can be delayed so that it causes the comb line (f2(t) = -f1(t - dn(λ m ) / c)) reverses the linewidth broadening of only one comb line in the output Fabry-Perot etalon. The output Fabry-Perot etalon (identical to the first etalon) then attenuates the optical power of the broadened comb line while efficiently transmitting a single narrow comb line. By controlling the drive signal supplied to the second modulator, any one of the comb lines can be selected for high transmission. Expanding on this concept, a dynamic sequence of comb line transmissions can be generated by applying a suitably constructed drive signal to the second phase modulator. This dynamic comb line transmission filter can be used to create a stepped-frequency comb laser.

[0031] Laser architecture

[0032] To construct the laser, Figure 1 The electrically controlled phase coding filter is placed in the ring cavity ( Figure 2 (a)). In this cavity, light travels from the filter output to the filter input without undergoing significant phase / spectral modulation. Therefore, the filter ( Figure 1 The two Fabry-Perot etalons used in (b)) are redundant and can be replaced by a single etalon. A fixed 80 GHz free spectral range (FSR) Fabry-Perot etalon (optomechanical) with a finesse of 100 is used. The phase-coded filter consists of two lithium niobate phase modulators (Covega) with a 10 GHz RF bandwidth. An arbitrary waveform generator (Euvis, AWG872) provides drive signals to these modulators through an RF amplifier. A dispersion-compensating fiber (OFS, WBDK:84C-L) provides -84 ps / nm dispersion at 1550 nm between the phase modulators. In order to equalize the entire cavity round-trip time over the wavelength range (dispersion matching), approximately 2.4 km of SMF-28e+ is inserted into the cavity in a double-pass configuration using a Faraday rotator mirror (FRM). The FRM eliminates the polarization mode dispersion of the SMF-28e+ fiber. A semiconductor optical amplifier (SOA, Covega) is located within the cavity for amplification, followed by an 80 / 20 output coupler.

[0033] Driving waveform

[0034] The drive waveforms provided to the first and second phase modulators set the filter transfer characteristics over time. Once the first modulator waveform is defined, the second modulator waveform is found by applying an appropriate delay relative to the inverted first waveform. There are many waveforms that can be used to drive the first modulator. In the embodiments presented herein, a chirped sine is used as the basis function for the first modulator waveform, although other waveforms can also be used. Other possible waveforms include pseudo-randomly generated binary (e.g., digital) and / or analog signals, or truly orthogonal codes commonly used in code division multiple access (CDMA). Truly orthogonal CDMA codes include, for example, Walsh codes, Walsh-Hadamard codes, and Gold codes. The logical basis for using a chirped sine in the embodiments presented herein is that the chirped sine lacks a dominant frequency component. The dominant frequency creates periodicity (i.e., an autocorrelation function) in the delayed response, thereby allowing multiple comb lines to be transmitted through the filter.

[0035] For the first modulator, a repetitive waveform is constructed to be provided to the first modulator (f1(t)). This waveform is constructed by concatenating sinusoidal signals that chirp from 1.0 GHz to 1.9 GHz or from 1.2 GHz to 2.3 GHz. Each chirp has a duration of t. p Execute on, where t p is the desired output pulse width (the time the laser output remains fixed at a given optical frequency). The chirped sine wave repeats indefinitely at the first modulator so that each output pulse is "encoded" with the same phase modulation.

[0036] The waveform provided to the second modulator is then constructed by connecting segments of the chirped sinusoids used to form the first waveform. These chirped sinusoids are inverted and delayed based on the desired output wavelength sequence. More specifically, a sequence of laser output comb line frequencies is first defined. Next, an optical group delay table is calculated for each optical comb line frequency in the sequence. Finally, these delays are used to construct the second phase modulator drive signal as described above. Interpolation is used to connect the chirped sinusoidal waveforms ( Figure 2 (b)). When the inter-modulator dispersion is -84 ps / nm, the optical group delay difference between adjacent comb lines (80 GHz free spectral range) is approximately 54 ps. Because this is significantly smaller than the AWG clock period (125 ps), we create nearly arbitrary delays in the waveform by generating appropriately different discrete representations of the chirped waveform and sending these new discrete representations to the digital-to-analog converter. This allows us to shift the delay of the generated signal with a delay accuracy less than the AWG clock period, as is known in signal sampling and generation.

[0037] The digital representation of the chirped sinusoid is phase-shifted to induce a delay with sub-clock cycle accuracy. For example, it is well known that a digital-to-analog converter with a sampling frequency F can be used at the Nyquist frequency F NY =F / 2 or below. There are two strategies to shift the phase of the signal at f. The first strategy is to delay (shift) the output signal by an integer number of clock cycles. This induces πn(f / F NY ) where n is an integer. For example, at f = F NY This method can be used to set the phase of the signal to 0 or π. NY The phase of the signal can be set to 0, π / 2, π, 3π, 2. A second strategy for shifting the phase is to drive the DAC with different samples of a given frequency, with both the first modulator waveform and the second modulator waveform designed so that the combline transmission filter operates at resonance with the cavity round-trip time, similar to the operation of Fourier domain mode locking.

[0038] Phase-coded filter performance

[0039] Before fabricating a prototype PCML laser, the performance of the phase-coded filter was first characterized in isolation, i.e., outside of its integration into the laser cavity. Figure 1 The setup shown in (b) is completed. Amplified spontaneous emission (ASE) light is used as input from the SOA. This light is transmitted through an 80 GHz Fabry-Perot etalon with a finesse of about 100. A second, identical etalon is placed at the output and the angle is adjusted so that its comb lines are aligned with the comb lines of the first etalon. Using an optical spectrum analyzer (Yokogawa, AQ6370C), the line width of each comb line is characterized as it passes through the filter. Due to the resolution limitation of the OSA, line widths below 0.02 nm cannot be resolved. Figure 3 In the measurements shown, the second modulator waveform was configured to decode the comb lines at 1560 nm. Note that the first phase modulator uniformly broadens all input comb lines ( Figure 3 (b)), but only the comb line at 1560 nm is significantly narrowed after the second phase modulator ( Figure 3 (c)). Next, the transmission efficiency of each comb line was measured using an OSA. As expected, the transmission efficiency of the narrow comb line is high ( Figure 3 (h)). The filter extinction of other lines is greater than 3.5dB ( Figure 3 (h)).

[0040] PCML laser performance

[0041] Next, the filter is incorporated into Figure 2The ring cavity is shown and operated at three speeds, specifically 176 kHz (5th harmonic of the cavity), 881 kHz (25th harmonic) and 3.52 MHz (100th harmonic), the speeds chosen being to recover multiple output bands at frequencies that are integer multiples of the round-trip frequency of the optical resonator.

[0042] Figure 4 Laser spectra and time traces of the PCML laser are shown for each of these configurations. At 176 kHz, a pulse width of 43 ns was used. At 881 kHz, a pulse width of 8.6 ns was used. For both speeds, the laser output spanned 80 nm at an 80 GHz comb line spacing. In a prototype laser using an 8 GSPS AWG, consistent performance could not be achieved below a pulse width of 8.6 ns. To achieve a speed of 3.52 MHz while maintaining a pulse width of 8.6 ns, the number of output wavelengths was reduced by designing the waveform transmitted every fourth comb line. Note that these changes in laser performance require only changes to the AWG drive signal supplied to the phase modulator; no hardware modifications are required. This enables the generation of a CR-OCT source with wide tunability and rapid reconfigurability.

[0043] Although the worst-case single-pass extinction of the filter is about 3.5 dB, the laser output achieves much higher comb-line extinction due to the laser cavity resonance. Figure 4 The laser output spectrum shown in (c) shows a spectral extinction exceeding 25 dB. However, the instantaneous laser extinction is likely to be higher when considering that the duty cycle of the selected line is much lower than that of the offline line.

[0044] The PCML laser coherence length is measured by acquiring a fringe signal that is a function of the sample arm mirror position relative to the reference arm. 176kHz and 881kHz configurations were measured to analyze the relationship between pulse width and the coherence length of the source. The point spread function of the fringe signal was calculated with increasing order at the same recurring delay position. The axial resolution was measured from the obtained point spread function to be 17μm, which is consistent with or without a boosted SOA. For pulse widths of 43ns (176kHz A-line) and 8.6ns (881kHz A-line), the coherence length (6dB attenuation) is 85mm (mirror shift) ( Figure 5(a, c)). This will achieve an imaging range of 170mm using cyclic ranging with in-phase and orthogonal fringe signal detection that distinguishes between positive and negative delay space. Note that the coherence length is measured to be the same for both pulse widths. Although not explicitly measured, it is expected that changing the sequence of the output comb lines at the same pulse width (e.g. 881kHz and 3.52MHz output) will not affect the coherence length. Because the prototype laser output power is low (about 1mW), the PCML coherence length was also tested using a boosted SOA (outside the cavity). The boosted SOA increased the power to 50mW and reduced the coherence length to 50mm (100mm CR-OCT imaging range), as shown Figure 5 (b, d). This reduction is a result of linewidth broadening in the SOA. It was also observed that the acoustic pressure SOA increased the noise floor, which requires further examination.

[0045] CR-OCT imaging using PCML laser

[0046] CR-OCT images were acquired at each of three imaging speeds (176 kHz, 881 kHz, and 3.52 MHz). As expected, the recurrence depth / delay range of the 3.52 MHz image was different from that of the 176 kHz and 881 kHz images due to the generation of a 320 GHz frequency comb instead of an 80 GHz frequency comb. The laser was used in conjunction with a boosted SOA for the imaging experiments. The quadrature demodulation circuit described by Siddiqui et al. (“Compensation of spectral and rferrors in swept-source OCT for high extinction complex demodulation,” Optics Express 23(5), 5508–5520 (2015), incorporated herein by reference) was used to create the in-phase and quadrature fringes required for CR-OCT. The output fringes were directed to a balanced optical receiver (Thorlabs, PDB465C) for detection. The digitizer (Signatec, PX14400) acquired the signal from the detector at 250 MS / s, which is significantly faster than required to capture the 8.6 ns (3.52 MHz and 881 kHz) and 43 ns (176 kHz) pulse widths. Figure 6 Shows the CR-OCT image acquired by the IR detection card ( Figure 6 (a)-6(c)), and CR-OCT images of the finger averaged over 25 adjacent cross sections at each laser speed ( Figure 6(d)-6(e)). For the 176kHz and 881kHz configurations, the depth of the recirculating range is approximately 1.9mm, and for the 3.52MHz configuration, the depth of the recirculating range is approximately 0.48mm. Note that the total imaging depth range and the recirculating delay range are different parameters; the signal can be acquired throughout the imaging range, which is limited by the source coherence, but appears within a compressed image equal to the recirculating delay range.

[0047] A novel laser architecture has been demonstrated that provides a stepped-frequency comb output for moderate-speed CR-OCT. In addition to enabling CR-OCT at speeds ranging from 100 kHz to several MHz, the PCML laser design has several unique features. Demonstrated in operation at 3.52 MHz, the PCML laser allows the user to set specific comb lines, skip comb lines, or modify the sequence of generated comb lines. This is accomplished through the design of the drive waveform, without hardware modification, and can be used to create highly reconfigurable sources for CR-OCT. As with SPML lasers, the speed of PCML lasers is not directly limited by the amount of intracavity dispersion. This allows PCML lasers to operate at more moderate speeds. The upper speed limit for PCML lasers is currently undefined. In principle, high-speed operation is possible. However, generating the required high-bandwidth RF drive signal may be difficult, making the SPML architecture more suitable for ultrahigh-speed operation. Additional advantages of the PCML architecture include 100% duty cycle output and temporally linear output, without the need for k-clocking or k-space resampling after digitization. In contrast, the main drawback of current PCML lasers is their noise performance, which is significantly higher than that of mature OCT laser technology.

[0048] Reduction of relative intensity noise (RIN)

[0049] Certain embodiments of the present invention provide procedures for reducing noise in the disclosed sources. In certain embodiments, the disclosed PCML sources can impose high relative intensity noise (RIN) that can limit their utility in practice. Thus, the procedures disclosed herein can make the PCML highly stable by adjusting the drive waveform of the phase modulator (EOM) to generate discrete pulses at each wavelength (with a "laser off" time between pulses) and at specific pulse durations (pulse widths) that are related to the finesse of the etalon used in the cavity ( Figure 7 This paper empirically analyzes the laser noise of different waveform designs ( Figure 8 ). Also provided are system performances showing improvements ( Figure 9 ) and imaging ( Figure 10 ) results.

[0050] The noise reduction procedure can be performed using a system such as that described above (see, for example, Figure 8 (a)). One modification of the system is to test multiple different etalons with similar FSR and different fineness for noise analysis, where one of the etalons is selected for the final laser demonstration.

[0051] The driving waveform of the phase modulator is designed to include an “on” and “off” state for each pulse ( Figure 7 (a)). The concept of on and off states originates from the integration time (switching time) of the etalon, i.e. the range of group delay transit times for light of different optical frequencies through the etalon. This integration time is the inverse of the linewidth of each etalon comb line. When a light pulse with a duration less than the integration time passes through the etalon, it is stretched in time, resulting in a pulse with a duration of approximately the integration time. By designing an off time between the pulses of sufficient duration, temporally overlapping pulses at the output of the etalon can be avoided. Temporally overlapping pulses lead to laser instabilities and higher intensity noise. Based on this, the optimal on / off times are determined as a function of the etalon linewidth (which is a function of the etalon finesse and the FSR).

[0052] During the laser on period, the chirped waveforms disclosed above are used to control the phase modulators. However, to achieve stable lasing in multiple cavity round trips, only one chirped waveform is used instead of multiple waveforms. During the laser off period, a uniform (non-chirped) sinusoidal waveform with a different RF frequency is used on each phase modulator ( Figure 7 (b)). Various experiments were performed to find the optimal laser on / off times for etalons with different linewidths. During the waveform associated with the on-time, light pulses are generated at a selected wavelength determined by the waveform characteristics described above. The waveform associated with the off-time suppresses transmission of all comb lines and instantaneously turns off the laser.

[0053] In the first set of experiments, three different physical etalons were used, all with an FSR of 80-85 GHz and finesses of 150, 500, and 8000. These three physical etalons were used to create laser sources with four different effective finesses: 150, 300 (double pass of the 150 etalon), 500, and 8000. In the first experiment, a single laser line at approximately 1560 nm was produced, repeating over time, with the repeating lines sufficiently spaced apart in time and each line having a different "laser on" time. The laser RIN and FWHM pulse width were investigated to find the optimal PW for RIN.

[0054] In a second set of experiments, a dual-line laser at approximately 1560 nm and 1559 nm was repeatedly applied over time, with varying "laser off" times. The laser RIN was examined as a function of the off time to find the minimum off time required to achieve the optimal RIN level. After finding the optimal RIN conditions, additional experiments were conducted to demonstrate the improved RIN sensitivity of the modified PCML operation. The skin of a subject's finger was imaged using the same imaging optical system as above to provide a comparison of image quality.

[0055] First, experiments were conducted to determine the noise performance of the modified laser as a function of the "laser on" time. These measurements were made with the laser configured to repeatedly generate pulses of the same wavelength (1560 nm). The noise performance of the laser was determined as a function of time (determined by the output laser pulse width). The noise was characterized at all four standard effective finesse levels (150, 300, 500, 8000) and pulse widths from 0.134 ns to 300 ns (see Figure 8 (a) The laser off time for each configuration is 0.284 ns for finesse of 150, 300, and 500, and 0.568 ns for finesse of 8000.

[0056] For all tested configurations, no lasing was observed when the “laser on” time was too short ( Figure 8 (a) When the "laser on" time is long enough to achieve lasing, the lasing begins and becomes quite stable. As the "laser on" time is further increased, the pulse width becomes larger, the lasing becomes unstable, and the intensity noise increases significantly.

[0057] The observed pattern is that with increasing laser on-time / pulse width, there are no transitions from stable to unstable lasing, with the scaling factor for these transitions depending on the etalon finesse. This defines a strategy to optimize the laser configuration for stable lasing as a function of etalon finesse. The optimal operating condition for laser on-time is approximately given by the inverse of the linewidth of each etalon comb line, where the etalon linewidth is given by the etalon FSR divided by the finesse. Therefore, the optimal pulse width is approximately given by the etalon finesse divided by the etalon FSR.

[0058] Experiments were then conducted to determine the noise performance as a function of the "laser off" time. These measurements were performed in a laser configured to repeatedly generate pulses alternating between two wavelengths (1560 nm and 1559 nm), as indicated above. Figure 8(b), 8(c), 8(d), and 8(e). The laser's "laser off" time scale is 0.134 ns to approximately 20 ns. The "laser on" time for each configuration was chosen from previous results to provide stable lasing (1.88 ns for 150, 2.97 ns for 300, 5.38 ns for 500, and 80.6 ns for 8000 finesse).

[0059] Although the "laser on" time is within the stable range, when the "laser off" time is too short, both pulses show significant noise, meaning they are not well separated. Once the "laser off" time exceeds a certain duration, the pulses become stable, as in the previous experiment. The threshold "laser off" time for stable lasing is also proportional to the etalon integration time, which is determined by the linewidth. The RIN is also found to be more consistent with higher finesse, likely because the longer "laser on" time allows for more accurate and predictable extinction of the signal from the AWG.

[0060] Finally, the experiment to characterize the noise of the PCML laser generated a full frequency comb output of 110 wavelengths. The PCML-OCT laser was demonstrated using a 500 finesse etalon with an A-line rate of 1.16 MHz. For each of the 110 pulses within a 75 nm range centered at 1050 nm, the laser on and off times were set to 3.6 ns and 4.4 ns, respectively, with the settings derived from Figure 8 results.

[0061] When measured using a 2 GHz bandwidth detector and digitizer, the quadrature RIN for each pulse was less than 1%. The rest of the system, including the interferometry, detection section, and imaging microscope, was identical to the source disclosed above. System sensitivity reached 104 dB with a sampling arm power of 35 mW and a 6 dB roll-off depth of 74 mm. Based on the work disclosed above, it can be assumed that the sensitivity roll-off of PCML is even longer without a boosted SOA. Finger skin imaging also showed a significant improvement in signal-to-noise ratio of approximately 20 dB compared to previous work.

[0062] Since there is an optimal pulse width for a given etalon, the noise performance of the laser degrades when the pulse width is extended beyond this optimum to reduce imaging speed. In one embodiment, the laser can be configured for slower speeds by generating effectively long pulses at a given wavelength by repeating the same wavelength multiple times. In one example, a 5 ns pulse at 1550.00 nm can be repeated ten times, creating a pulse train of 10 individual pulses with a duration of 50 ns. The detection system can convert this pulse train into a single 50 ns pulse using low-pass analog filtering. In this way, the laser speed can be reduced while maintaining optimal noise performance. The number of repeated pulses at each wavelength can be any integer, subject to the constraint mentioned above that the laser output train is equal to or a harmonic of the cavity round-trip time.

[0063] Computer and optical systems

[0064] Steering Figure 11 In accordance with some embodiments of the disclosed subject matter, an example 1100 of a system (e.g., a data collection and processing system) for providing a source for cyclic ranging (CR-OCT) is shown. In some embodiments, a computing device 1110 can execute at least a portion of the system for providing a source for CR-OCT 1104 and provide control signals to one or more optical modulators associated with an optical resonator 1102. Additionally or alternatively, in some embodiments, the computing device 1110 can transmit information regarding the control signals to or from a server 1120 via a communication network 1106, which can execute at least a portion of the system for providing a source for CR-OCT 1104. In some such embodiments, the server 1120 can return information regarding the control signals for the system for providing a source for CR-OCT 1104 to the computing device 1110 (and / or any other suitable computing device). This information can be transmitted and / or presented to a user (e.g., a researcher, operator, clinician, etc.) and / or can be stored (e.g., as part of a research database or medical record associated with a subject).

[0065] In some embodiments, computing device 1110 and / or server 1120 can be any suitable computing device or combination of devices, such as a desktop computer, a laptop computer, a smartphone, a tablet computer, a wearable computer, a server computer, a virtual machine executed by a physical computing device, etc. As described herein, a system for providing a source for CR-OCT 1104 can present information about the control signal to a user (e.g., a researcher and / or a physician). In some embodiments, optical resonator 1102 can include optical components such as those disclosed herein (e.g., see Figure 2 (a)).

[0066] In some embodiments, the communication network 1106 may be any suitable communication network or combination of communication networks. For example, the communication network 1106 may include a Wi-Fi network (which may include one or more wireless routers, one or more switches, etc.), a peer-to-peer network (e.g., a Bluetooth network), a cellular network (e.g., a 3G network, a 4G network, etc., conforming to any suitable standard (such as CDMA, GSM, LTE, LTE Advanced, WiMAX, etc.)), a wired network, etc. Figure 11 The communication links shown may each be any suitable communication link or combination of communication links, such as a wired link, a fiber optic link, a Wi-Fi link, a Bluetooth link, a cellular link, or the like.

[0067] Figure 12 1 shows an example 1200 of hardware that can be used to implement computing device 1110 and server 1120 according to some embodiments of the disclosed subject matter. Figure 12 As shown, in some embodiments, computing device 1110 may include a processor 1202, a display 1204, one or more inputs 1206, one or more communication systems 1208, and / or memory 1210. In some embodiments, processor 1202 may be any suitable hardware processor or combination of processors, such as a central processing unit, a graphics processing unit, etc. In some embodiments, display 1204 may include any suitable display device, such as a computer monitor, a touch screen, a television, etc. In some embodiments, input 1206 may include any suitable input device and / or sensor that can be used to receive user input, such as a keyboard, a mouse, a touch screen, a microphone, etc.

[0068] In some embodiments, the communication system 1208 may include any suitable hardware, firmware, and / or software for transmitting information via the communication network 1106 and / or any other suitable communication network. For example, the communication system 1208 may include one or more transceivers, one or more communication chips and / or chipsets, etc. In more specific examples, the communication system 1208 may include hardware, firmware, and / or software for establishing a Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, etc.

[0069] In some embodiments, the memory 1210 may include any suitable storage device or device that can be used to store instructions, values, etc., which can be used, for example, by the processor 1202 to present content using the display 1204, to communicate with the server 1120 via the communication system(s) 1208, etc. The memory 1210 may include any suitable volatile memory, non-volatile memory, storage device, or any suitable combination thereof. For example, the memory 1210 may include RAM, ROM, EEPROM, one or more flash drives, one or more hard disks, one or more solid-state drives, one or more optical drives, etc. In some embodiments, the memory 1210 may have encoded thereon a computer program for controlling the operation of the computing device 1110. In such embodiments, the processor 1202 may execute at least a portion of the computer program to present content (e.g., images, user interfaces, graphics, tables, etc.), receive content from the server 1120, transmit information to the server 1120, etc.

[0070] In some embodiments, server 1120 may include a processor 1212, a display 1214, one or more inputs 1216, one or more communication systems 1218, and / or memory 1220. In some embodiments, processor 1212 may be any suitable hardware processor or combination of processors, such as a central processing unit, a graphics processing unit, etc. In some embodiments, display 1214 may include any suitable display device, such as a computer monitor, a touch screen, a television, etc. In some embodiments, input 1216 may include any suitable input device and / or sensor that can be used to receive user input, such as a keyboard, a mouse, a touch screen, a microphone, etc.

[0071] In some embodiments, the communication system 1218 may include any suitable hardware, firmware, and / or software for transmitting information via the communication network 1106 and / or any other suitable communication network. For example, the communication system 1218 may include one or more transceivers, one or more communication chips and / or chipsets, etc. In more specific examples, the communication system 1218 may include hardware, firmware, and / or software for establishing a Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, etc.

[0072] In some embodiments, the memory 1220 may include any suitable storage device or device that can be used to store instructions, values, etc., which can be used, for example, by the processor 1212 to present content using the display 1214, to communicate with one or more computing devices 1110, etc. The memory 1220 may include any suitable volatile memory, non-volatile memory, storage device, or any suitable combination thereof. For example, the memory 1220 may include RAM, ROM, EEPROM, one or more flash drives, one or more hard disks, one or more solid-state drives, one or more optical drives, etc. In some embodiments, the memory 1220 may encode thereon a server program for controlling the operation of the server 1120. In such embodiments, the processor 1212 may execute at least a portion of the server program to transmit information and / or content (e.g., organizational identification and / or classification results, user interface, etc.) to one or more computing devices 1110, receive information and / or content from one or more computing devices 1110, receive instructions from one or more devices (e.g., personal computers, laptop computers, tablet computers, smartphones, etc.), etc.

[0073] In some embodiments, any suitable computer-readable medium may be used to store instructions for performing the functions and / or processes described herein. For example, in some embodiments, the computer-readable medium may be transient or non-transient. For example, non-transient computer-readable media may include media such as magnetic media (such as hard disks, floppy disks, etc.), optical media (such as compact discs, digital video discs, Blu-ray discs, etc.), semiconductor media (such as RAM, flash memory, electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), etc.), any suitable medium that is not transient or does not have any persistent appearance during transmission, and / or any suitable tangible medium. As another example, transient computer-readable media may include signals on a network, in wires, conductors, optical fibers, circuits, or any suitable medium that is transient and does not have any persistent appearance during transmission, and / or any suitable intangible medium.

[0074] It should be noted that the term mechanism as used herein may encompass hardware, software, firmware, or any suitable combination thereof.

[0075] Figure 13 A schematic diagram of an interferometry system that may be used in conjunction with various embodiments of the present invention is provided. Figure 13 A Mach-Zehnder interferometer is shown, which can be used with free-space optics ( Figure 13 , panel A) or fiber optic arrangement ( Figure 13Other types of interferometers (e.g., Michelson) can also be used. Figure 13 The light source LS in panel A or panel B of can be an embodiment of a PCML laser as disclosed herein. The beam B9 emitted from LS is directed to the interferometer input where it is split into two paths of approximately equal length using a beam splitter (BS3). B10 is directed towards the sample S. The backscattered light from the object of interest is then directed towards the interferometer output (B11). In the reference arm, beam B12 is directed towards the phase modulator (PM). The beam after PM (i.e., beam B13) is directed to the interferometer output to interfere with beam B11 after being combined by BS4. The output beam B14 is then detected by a detector D (e.g., a photodiode). Alternatively, Figure 13 The fiber-based interferometer shown in panel B of FIG1 easily allows for balanced detection due to the phase shift of π between the output beams B14 and B15. Using a data collection and processing system (which may include a data acquisition board or a real-time oscilloscope (DAQ)) with a sampling rate of f S The detected signal is digitized. Several wavelength sweeps (A1, A2, ..., An) can be acquired to form a two-dimensional or three-dimensional image.

[0076] Therefore, although the invention has been described above in conjunction with particular embodiments and examples, the invention is not necessarily so limited, and many other embodiments, examples, uses, modifications and deviations from the described embodiments, examples and uses are intended to be encompassed by the appended claims.

Claims

1. A source for providing electromagnetic radiation within a specific spectral range, the source comprising: A ring optical resonator, wherein the ring optical resonator is used to circulate multiple bands, and the ring optical resonator comprises: a first optical phase modulator, First dispersion device, a second optical phase modulator, Multi-line spectral domain filters, a second dispersion device, and Optical amplifiers; a controller coupled to the first optical phase modulator and the second optical phase modulator, the controller configured to drive the first optical phase modulator with a first waveform and to drive the second optical phase modulator with a second waveform, The first dispersive device is arranged between the first optical phase modulator and the second optical phase modulator to provide dispersion so that each of the plurality of wavelength bands is subjected to a corresponding plurality of different time delays, The first optical phase modulator and the second optical phase modulator are configured to create spectral broadening for each of the plurality of wavelength bands by the first optical phase modulator, perform spectral recovery for a specific wavelength band of the plurality of wavelength bands by the second optical phase modulator by modulating a first phase using the first optical phase modulator driven by the first waveform, and modulate a second phase after a specific time delay using the second optical phase modulator driven by the second waveform including an inverse of the first waveform, determining the specific time delay to create a spectral restoration for the specific wavelength band of the plurality of wavelength bands, The multi-line spectral domain filter is configured to provide multi-line spectral filtering with a narrow bandwidth so as to induce power loss for each of the plurality of wavelength bands except the specific wavelength band, The second dispersive device is configured to provide dispersion compensation to the output of the multi-line spectral domain filter to compensate for group delay dispersion within the optical resonator and match the round-trip frequency of each of the plurality of bands, and The first waveform and the second waveform are configured to create a periodic phase modulation to recover the plurality of bands at frequencies that are integer multiples of a round-trip frequency of the optical resonator.

2. The source of claim 1, wherein The optical resonator further includes an optical isolator configured to provide optical isolation so that laser light travels in one direction in the optical resonator.

3. A source as claimed in any one of claims 1 or 2, wherein At least one of the first optical phase modulator and the second optical phase modulator comprises an electro-optical phase modulator.

4. The source of claim 3, wherein At least one of the first optical phase modulator and the second optical phase modulator comprises a lithium niobate phase modulator.

5. The source of any one of claims 1 or 2, wherein The first dispersive device is a dispersive optical fiber, a chirped fiber Bragg grating, a fiber Bragg grating array or a reflective optical fiber delay line.

6. The source of claim 5, wherein The second dispersive device is a dispersive optical fiber, a chirped fiber Bragg grating, a fiber Bragg grating array or a reflective optical fiber delay line.

7. The source of claim 6, wherein The first dispersive device is a different type of device than the second dispersive device.

8. The source of any one of claims 1 or 2, wherein The multi-line spectral domain filter includes a Fabry-Perot etalon.

9. The source of any one of claims 1 or 2, wherein The optical amplifier is a semiconductor optical amplifier or an erbium-doped fiber amplifier.

10. The source of any one of claims 1 or 2, wherein The first dispersive device provides anomalous dispersion, and the second dispersive device provides normal dispersion.

11. The source of any one of claims 1 or 2, wherein The first dispersive device provides normal dispersion, and the second dispersive device provides anomalous dispersion.

12. The source of any one of claims 1 or 2, wherein The controller includes a dual-channel arbitrary waveform generator.

13. The source of claim 12, further comprising an RF amplifier for amplifying the first waveform and the second waveform.

14. The source of claim 12, wherein The first and second waveforms are generated from different discrete representations of the first and second waveforms, such that a delay between the first and second waveforms can be controlled to a precision higher than a digital-to-analog sampling frequency.

15. The source of any one of claims 1 or 2, wherein The optical resonator further includes an output coupler configured to emit first output electromagnetic radiation based on the plurality of wavelength bands.

16. The source of any one of claims 1 or 2, wherein The specific time delay is adjusted so that each of the plurality of wavelength bands is spectrally restored by the second optical phase modulator in the order of wavelengths to generate a wavelength-stepped laser.

17. The source of any one of claims 1 or 2, wherein At least one of the first waveform or the second waveform comprises a chirped sinusoidal waveform.

18. The source of any one of claims 1 or 2, wherein The first waveform and the second waveform are configured to create a recovered periodic phase modulation for the plurality of bands of frequencies between 100 kHz and 5 MHz.

19. The source of any one of claims 1 or 2, wherein The source operates during laser-on and laser-off periods.

20. The source of claim 19, wherein At least one of the first waveform or the second waveform includes a chirped sine during the laser on period and a uniform sine waveform during the laser off period.

21. The source of any one of claims 1 or 2, wherein The source generates light pulses having a duration (pulse width) related to the inverse of the linewidth of the Fabry-Perot transmission peak.

22. A source for providing electromagnetic radiation within a specific spectral range, the source comprising: A ring optical resonator, wherein the ring optical resonator is used to circulate multiple bands, and the optical resonator comprises: a first optical phase modulator, dispersion equipment, a second optical phase modulator, Multiline spectral domain filters, and Optical amplifiers; a controller coupled to the first optical phase modulator and the second optical phase modulator, the controller configured to drive the first optical phase modulator with a first waveform and to drive the second optical phase modulator with a second waveform, The dispersion device is arranged between the first optical phase modulator and the second optical phase modulator to provide the dispersion so that each of the plurality of wavelength bands is subjected to a corresponding plurality of different time delays, The first optical phase modulator and the second optical phase modulator are configured to create spectral broadening for each of the plurality of wavelength bands by the first optical phase modulator, perform spectral recovery for a specific wavelength band of the plurality of wavelength bands by the second optical phase modulator by modulating a first phase using the first optical phase modulator driven by the first waveform, and modulate a second phase after a specific time delay using the second optical phase modulator driven by the second waveform including an inverse of the first waveform, determining the specific time delay to create a spectral restoration for the specific wavelength band of the plurality of wavelength bands, The multi-line spectral domain filter is configured to provide multi-line spectral filtering with a narrow bandwidth so as to induce power loss for each of the plurality of wavelength bands except the specific wavelength band, The first waveform and the second waveform are configured to create a recovery of sufficiently long duration that each of the plurality of wavelength bands undergoes multiple round trips within the optical resonator, and The first waveform and the second waveform are configured to create a periodic phase modulation to recover the plurality of bands at frequencies that are integer multiples of a round-trip frequency of the optical resonator.

23. The source of claim 22, wherein The optical resonator further includes an optical isolator configured to provide optical isolation so that laser light travels in one direction in the optical resonator.

24. The source of any one of claims 22 or 23, wherein At least one of the first optical phase modulator and the second optical phase modulator comprises an electro-optical phase modulator.

25. The source of claim 24, wherein At least one of the first optical phase modulator and the second optical phase modulator comprises a lithium niobate phase modulator.

26. The source of any one of claims 22 or 23, wherein The dispersion device is a dispersion optical fiber, a chirped fiber Bragg grating, a fiber Bragg grating array or a reflective fiber delay line.

27. The source of any one of claims 22 or 23, wherein The multi-line spectral domain filter includes a Fabry-Perot etalon.

28. The source of any one of claims 22 or 23, wherein The optical amplifier is a semiconductor optical amplifier or an erbium-doped fiber amplifier.

29. The source of any one of claims 22 or 23, wherein The controller includes a dual-channel arbitrary waveform generator.

30. The source of claim 29, further comprising an RF amplifier for amplifying the first waveform and the second waveform.

31. The source of claim 29, wherein The first waveform and the second waveform are generated by performing phase shifting in the frequency domain to improve the accuracy of the specific time delay.

32. The source of any one of claims 22 or 23, wherein The optical resonator further includes an output coupler configured to emit first output electromagnetic radiation based on the plurality of wavelength bands.

33. The source of any one of claims 22 or 23, wherein The specific time delay is adjusted so that each of the plurality of wavelength bands is spectrally restored by the second optical phase modulator in the order of wavelengths to generate a wavelength-stepped laser.

34. The source of any one of claims 22 or 23, wherein At least one of the first waveform or the second waveform comprises a chirped sinusoidal waveform.

35. The source of any one of claims 22 or 23, wherein The first waveform and the second waveform are configured to create a recovered periodic phase modulation for the plurality of bands of frequencies between 100 kHz and 5 MHz.

36. The source of any one of claims 22 or 23, wherein The source operates during laser-on and laser-off periods.

37. The source of claim 36, wherein At least one of the first waveform or the second waveform includes a chirped sine during the laser on period and a uniform sine waveform during the laser off period.

38. The source of any one of claims 22 or 23, wherein The source generates light pulses having a duration (pulse width) that is related to the inverse of the linewidth of the Fabry-Perot transmission peak.

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