Frequency-comb generation based on electro-optic phase-code mode-locking for circular-ranging oct

Abstract

To provide mechanisms for providing a source for circular-ranging OCT at moderate speeds.SOLUTION: A source for providing electromagnetic radiation within a particular spectral range includes: a ring-shaped optical resonator for circulating a plurality of wavelength bands including a first optical phase modulator, a first chromatic dispersion device, a second optical phase modulator, a multi-line spectral domain filter, a second chromatic dispersion device, and an optical amplifier; and 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 the second optical phase modulator with a second waveform.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] <CROSS REFERENCE TO RELATED APPLICATIONS> This application is based on and claims priority to U.S. Provisional Patent Application No. 62 / 968,299, filed January 31, 2020, the entire disclosure of which is incorporated herein by reference.

[0002] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT: This invention was made with government support under Grant No. P41EB015903 awarded by the National Institutes of Health and Grant No. FA9550-11-1-0331 awarded by the Air Force Office of Scientific Research. The U.S. Government has certain rights in this invention. [Background technology]

[0003] Optical coherence tomography (OCT) is widely used to image the three-dimensional structure of biological tissues and materials. A distinctive feature of OCT technology is the use of echo delay ranging to resolve the depth location of scatterers. Recently described circular ranging Range-resolved (CR) OCT utilizes a compressed echo delay ranging approach to reduce the number of measurements required to capture a signal with sparsity in the depth domain. This sparsity is common in OCT, especially in long-range OCT applications. Reducing the number of measurements reduces the bandwidth of electronic signal capture and processing, leading to high-speed imaging for a given electronic bandwidth, or simplified signal capture systems for medium-speed imaging. To image at the highest possible speeds, stretched pulse mode-locked (SPML) lasers provide the stepped frequency comb output used in CR-OCT at repetition rates of several megahertz to tens of megahertz. These SPML sources do not provide a solution for medium-speed CR-OCT because they cannot be easily scaled to lower speeds. Instead, a different source technology is needed to enable medium-speed CR-OCT. Summary of the Invention

[0004] Therefore, new systems, methods and apparatus for providing a circular ranging OCT source are desired.

[0005] In one embodiment, the present invention provides a source for providing electromagnetic radiation within a particular spectral range, the source comprising a ring-shaped optical resonator for circulating a plurality of wavelength bands. the optical resonator including a first optical phase modulator, a first wavelength dispersion device, a second optical phase modulator, a multi-line spectral domain filter, a second wavelength dispersion device, and an optical amplifier; and a controller connected 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 first wavelength dispersion device being disposed between the first optical phase modulator and the second optical phase modulator and providing wavelength dispersion such that each of a plurality of wavelength bands experiences a respective plurality of different time delays, The second optical phase modulator is configured to modulate a first phase using a first optical phase modulator driven by a first waveform, and modulate a second phase using a second optical phase modulator driven by a second waveform, the second waveform comprising an inverse waveform of the first waveform, after a specific time delay, to produce spectral broadening by the first optical phase modulator for each of a plurality of wavelength bands and spectral restoration by the second optical phase modulator for a particular wavelength band of the plurality of wavelength bands, the specific time delay being determined to produce spectral restoration for a particular wavelength band of the plurality of wavelength bands, and a multi-line spectral domain filter. The filter is configured to provide multi-line spectral filtering with a narrow bandwidth to cause a power loss in each of a plurality of wavelength bands other than the particular wavelength band, and the second chromatic dispersion device is configured to chromatic dispersion compensate an output of the multi-line spectral domain filter to compensate for group delay dispersion in the optical cavity to match a roundtrip frequency for each of the plurality of wavelength bands, and the first waveform and the second waveform is configured to produce a periodic phase modulation for recovering a plurality of wavelength bands at a frequency that is an integer multiple of the roundtrip frequency of the optical resonator.

[0006] In another embodiment, the present invention provides a source for providing electromagnetic radiation within a particular spectral range, the source comprising: a ring-shaped optical resonator for circulating a plurality of wavelength bands, the optical resonator including a first optical phase modulator, a wavelength dispersion device, a second optical phase modulator, a multi-line spectral domain filter, and an optical amplifier; and a controller connected 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 wavelength dispersion device being disposed between the first optical phase modulator and the second optical phase modulator to provide wavelength dispersion such that each of the plurality of wavelength bands undergoes a respective plurality of different time delays, the first optical phase modulator and the second optical phase modulator modulating a first phase using the first optical phase modulator driven by the first waveform, and after a particular time delay, modulating a second waveform comprising an inverse waveform of the first waveform. a second optical phase modulator driven by a first optical phase modulator configured to modulate the second phase to cause spectral broadening by the first optical phase modulator for each of the multiple wavelength bands and to cause spectral restoration by the second optical phase modulator for a specific wavelength band of the multiple wavelength bands, a specific time delay is determined to cause spectral restoration for a specific wavelength band of the multiple wavelength bands, the multi-line spectral domain filter is configured to provide multi-line spectral filtering with a narrow bandwidth to cause power loss in each of the multiple wavelength bands other than the specific wavelength band, the first waveform and the second waveform are configured to cause restoration having a duration sufficiently long such that each of the multiple wavelength bands makes multiple revolutions within the optical resonator, and the first waveform and the second waveform are configured to cause periodic phase modulation to restore the multiple wavelength bands at a frequency that is an integer multiple of a revolution frequency of the optical resonator.

[0007] Various objects, features and advantages of the disclosed subject matter will become more fully apparent from the following detailed description of the disclosed subject matter when considered in conjunction with the following drawings, in which like reference characters refer to like elements and in which: [Brief description of the drawings]

[0008] [Figure 1] Fig. 1 shows the working principle of the phase-encoded wavelength filter. (a) shows the linewidth broadening and restoration of narrowband light by phase modulation and compensation. (b) shows the selective linewidth restoration of a frequency comb by using the dispersion between two phase modulators and wavelength filtering using a Fabry-Perot etalon. PM is a phase modulator, FG is a function generator, and SOA is a semiconductor optical amplifier. [Diagram 2] (a) shows the setup of the PCML laser, and (b) illustrates the RF waveform design of the phase modulator. AWG is an arbitrary waveform generator, PM is a phase modulator, PC is a polarization controller, FPE is a Fabry-Perot etalon, DCF is a dispersion compensating fiber, FRM is a Faraday rotator mirror, SMF is a single mode fiber, SOA is a semiconductor optical amplifier, and ISO is an isolator. [Diagram 3] Single-pass phase-encode filter performance measurements of each comb line are shown. The linewidth ((a), (c), (e), (g)) and intensity ((b), (d), (f), (h)) were measured after passing through the first etalon ((a), (b)), after linewidth broadening by the first modulator ((c), (d)), after linewidth restoration by the second modulator ((e), (f)), and after filtering by the second etalon ((g), (h)). [Figure 4] Laser spectra (left), spectra (center) and time traces (right) for the wavelength range indicated by the red box on the left are shown for PCML lasers operating at (a) 176 kHz, 130 wavelengths, tp=43 ns, (b) 881 kHz, 130 wavelengths, tp=8.6 ns, and (c) 3.52 MHz, 31 wavelengths, tp=8.6 ns. [Diagram 5] The 6 dB roll-off measurements of the PCML laser are shown for various settings: (a) 176 kHz without booster SOA, (b) 176 kHz with booster SOA, (c) 881 kHz without booster SOA, and (d) 881 kHz with booster SOA. [Figure 6](a)-(c) show cross-sectional images of an IR card, and (d), (e) show cross-sectional images of a finger with different A-line configurations at the same scale and corresponding circular range. Data were collected at three different imaging rates: (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 (a): 200 μm. [Figure 7] (a) shows the PCML A-line architecture with laser on / off operation, and (b) shows the AWG waveform design of each modulator for laser on / off operation. [Figure 8] (a) shows the orthogonal RIN of the single-pulse experiment, where the red / blue / green / purple dots correspond to 150 / 300 / 500 / 8000 finesse at 1560 nm, respectively, and (b)-(e) show the orthogonal RIN plots of the double-pulse experiment for 150 / 300 / 500 / 8000 finesse, respectively. The blue dots correspond to the first-come 1560 nm and the orange dots correspond to the trailing 1559 nm. [Figure 9] (a) shows the OSA spectrum of the PCML laser, and (b) shows the time trace and orthogonal RIN at the peak of each pulse. [Figure 10] (a) shows an unaveraged cross-sectional image of a finger acquired from the PCML-OCT system, and (b) shows a cross-sectional image of a finger at the same position averaged over 25 out-of-plane images. [Figure 11] 1 illustrates an example of a system for providing a circular ranging OCT source according to some embodiments of the disclosed subject matter. [Figure 12] 1 illustrates example hardware that can be used to implement computing devices and servers according to some embodiments of the disclosed subject matter. [Figure 13]1A-1B illustrate an interferometer system that can be used in conjunction with various embodiments disclosed herein, where (A) shows a Mach-Zehnder interferometer that can be implemented using free-space optics and (B) shows a fiber arrangement. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] In accordance with some embodiments of the disclosed subject matter, mechanisms (which may include systems, methods, and devices) are provided for providing a circular ranging OCT source, particularly at moderate speeds.

[0010] Circular ranging (CR) optical coherence tomography (OCT) uses frequency comb sources to improve long-distance imaging. While early development of CR-OCT focused on ultrafast imaging (i.e., operation at A-line rates of several MHz to tens of MHz), there are many applications and imaging strategies where more moderate speeds are preferred. However, there are no suitable frequency comb sources that enable moderate speed CR-OCT imaging. Described herein are embodiments of a novel phase-coded mode-locked (PCML) laser architecture that can operate in the kilohertz to megahertz range while also offering novel features such as dynamic reconfigurability and simplified time-linear frequency stepping. A prototype CR-OCT system with a PCML laser is presented, and imaging results are shown at A-line rates of 176 kHz to 3.52 MHz with a coherence-length-limited imaging depth of 170 mm. Various embodiments of the disclosed device include: A-line rates ranging from 100 kHz to 5 MHz can be achieved. In various further embodiments of the disclosed apparatus, a sequence of optical pulses can be achieved on the optical frequency comb line, each pulse having a pulse width of 1 ns to 100 ns.

[0011] Optical coherence tomography (OCT) is a widely used three-dimensional imaging modality defined by the use of echo-delay depth ranging. Recently described circular ranging (CR) OCT techniques use a stepped-in-time frequency comb source. Compressed echo delay ranging is possible using a wide variety of comb sources. With compressed ranging, fewer measurements are needed to interrogate long depth ranges. This reduces the bandwidth requirements of the electronics used to capture and process the output signal. CR-OCT at speeds above 10 MHz has been demonstrated using a fast stepped frequency comb source based on stretched pulse mode-locking (SPML).

[0012] The compression provided by CR can also be used to reduce the bandwidth requirements of the electronics in more moderate speed long range imaging applications. Unfortunately, SPML lasers are not well suited for medium speed imaging, and very long chirped fiber Bragg gratings are required to reduce speeds below a few megahertz. Medium speed CR-OCT instead relies on sources created by modifying existing swept source laser architectures. A fixed Fabry-Perot etalon is added to a polygon mirror and a micromechanical Fabry-Perot swept wavelength laser. This provides the required stepped frequency comb output, but the noise and coherence length characteristics are relatively poor, and the time nonlinear output pulse sequence complicates signal capture.

[0013] Thus, disclosed herein are embodiments of a novel stepped-frequency comb laser architecture for use in CR-OCT. This laser architecture is called phase-coded mode-locked (PCML). Similar to SPML lasers, PCML lasers use intracavity dispersion and electro-optic modulation instead of mechanically tuned spectral filters. The difference between SPML and PCML architectures is that SPML architectures use electro-optic amplitude modulation, whereas PCML architectures use electro-optic phase modulation. This difference, together with the use of appropriately designed control signals, allows the repetition rate of the laser to be decoupled from the intracavity dispersion. As a result, the PCML laser embodiments disclosed herein are capable of operating at moderate speeds with moderate levels of intracavity dispersion. Furthermore, PCML sources can operate at a wide range of speeds without hardware modifications and can generate nearly arbitrary comb line sequences (i.e., order of output wavelengths in time).

[0014] PCML lasers exploit reversible electro-optic linewidth broadening to create a frequency comb line transmission filter. The principle is relatively simple and is illustrated in Figure 1. As a starting point, the variation of a narrowband CW input field is first analyzed at optical frequency ω, passing through two successive phase modulators (Figure 1(a)), E A The light is emitted at point A. The first modulator is driven by a voltage drive signal f(t), and E B We obtain an output field given by (t) = cos(ωt) + αf1(t), where α is the scale factor (rad / V) between the induced phase shift in the modulator and the driving voltage. For simplicity, we assume that α is constant at RF and optical frequencies.

[0015] This phase modulation broadens (broadens) the linewidth of the light at point B, as shown in Figure 1(a). The spectrally broadened light travels from the first modulator to a second phase modulator at a distance d (into the fiber, at point C). The field at this point is E C(t) = cos(ω(t-dn / c) + αf1(t-dn / c)), where n is the group index of the fiber and c is the speed of light. At the second phase modulator output (point D), the field is E D If the drive signal is configured such that f2(t)=-f1(t-dn / c), the two modulations are cancelled, restoring the original narrowband linewidth and reducing E D It is easy to see that (t) = cos(ωt). Note that this reversible linewidth broadening only occurs if the delay of signal f2 relative to f1 matches the optical group delay between the modulators.

[0016] In FIG. 1(b), a comb line transmission filter is constructed based on this 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. When broadband (e.g., amplified spontaneous emission (ASE)) light is emitted, a frequency comb is generated by the first etalon. Each line of this frequency comb is equally broadened by the first phase modulator. Now, due to the presence of the dispersive fiber, each optical comb line experiences a different group delay during its journey to the second modulator. The drive signal f2 of the second modulator can be delayed to reverse the linewidth broadening of one of the comb lines (f2(t)=-f1(t-dn(λ m ) / c)). The output Fabry-Perot etalon (same as the first etalon) then attenuates the optical power of the broad comb lines while efficiently transmitting the single narrow comb line. By controlling the drive signal provided to the second modulator, any comb line can be selected for high transmission. This concept can be extended to generate a dynamic sequence of comb line transmissions by applying an appropriately configured drive signal to a second phase modulator. This dynamic comb line transmission filter can be used to create a stepped frequency comb laser.

[0017] <Laser Architecture> To construct the laser, the electronically controlled phase-code filter of Figure 1 was placed in a ring cavity (Figure 2(a)). In this cavity, light proceeds from the filter output to the filter input without significant phase / spectral modulation. As a result, the two Fabry-Perot etalons used in the filter (Figure 1(b)) are redundant and can be replaced by a single etalon. A fixed 80 GHz free spectral range (FSR) Fabry-Perot etalon (Light Machinery) with a finesse of 100 was used. The phase-code filter contained two lithium niobate phase modulators (Covega) with 10 GHz RF bandwidth. An arbitrary waveform generator (Euvis, AWG872) provided the drive signals to these modulators via RF amplifiers. A dispersion compensating fiber (OFS, WBDK:84C-L) provided a dispersion of -84 ps / nm at 1550 nm between the phase modulators. To balance the cavity roundtrip time across wavelengths, For equalization (dispersion matching), a length of approximately 2.4 km of SMF-28e+ was inserted into the cavity in a double-pass configuration using a Faraday rotator mirror (FRM). The FRM eliminated the polarization mode dispersion of the SMF-28e+ fiber. A semiconductor optical amplifier (SOA, Covega) was placed in the cavity for amplification, followed by an 80 / 20 output coupler.

[0018] <Driving waveform> The filter transmission characteristics are set over time by the drive waveforms provided to the first and second phase modulators. Once the waveform of the first modulator is defined, the second waveform is set by applying an appropriate delay to the inverted first waveform. There are many waveforms that can be used to drive the first modulator. In the embodiments shown herein, a chirp sine wave is used as the basis function for the first modulator waveform, although other waveforms may be used. Other possible waveforms include pseudo-randomly generated binary (e.g., digital) and / or analog signals, or true orthogonal codes commonly used in code division multiple access (CDMA). True orthogonal CDMA codes include, for example, Walsh codes, Walsh-Hadamard codes, and Gold codes. In the embodiments shown herein, a chirp sine wave is used as the basis function for the first modulator waveform. The reason for using a chirp wave is that a chirp sine wave lacks a dominant frequency component, which would cause periodicity in the delay response (i.e., an autocorrelation function) and therefore multiple comb lines may be transmitted through the filter.

[0019] For the first modulator, a repeating waveform was constructed that was provided to the first modulator (f1(t)). This waveform was constructed by combining sine waves chirped from 1.0 GHz to 1.9 GHz or from 1.2 GHz to 2.3 GHz. Each chirp was of duration t p where t p is the desired output pulse width (the amount of time the laser output remains fixed at a particular optical frequency). The chirped sine wave was repeated indefinitely in the first modulator so that each output pulse was "encoded" with the same phase modulation.

[0020] The waveform provided to the second modulator was then constructed by combining sections of the chirped sine waves used to form the first waveform. These chirped sine waves were inverted and delayed based on the desired output wavelength sequence. More specifically, the comb line frequency sequence of the laser output was first defined. Then, a table of optical group delays for each optical comb line frequency in this sequence was calculated. Finally, these delays were used to construct the drive signal for the second phase modulator mentioned above. The chirped sine wave waveforms were connected using interpolation (Figure 2(b)). With a dispersion between the modulators of -84 ps / nm, the optical group delay difference between adjacent comb lines (80 GHz free spectral range) was about 54 ps. Since this is considerably smaller than the clock cycle of the arbitrary waveform generator (125 ps), we induced a nearly arbitrary delay in the waveform by generating appropriately different discrete representations of the chirped waveform and sending this new discrete representation to a digital-to-analog converter. In this way, and as is known in signal sampling and generation, the delay of the generated signal could be shifted with a delay precision smaller than the clock cycle of the arbitrary waveform generator.

[0021] The digital representation of the chirp sine wave is phase shifted to introduce a delay with sub-clock cycle accuracy. For example, a digital-to-analog converter with sampling frequency F can be used to generate a digital representation of the chirp sine wave with a Nyquist frequency F. NY It is known that it is possible to generate signals with various phases up to πn(f / F). There are two strategies to shift the phase of a signal by f. The first is to delay (shift) the output signal by an integer number of clock cycles. This allows us to NY ), where n is an integer. For example, f=F NY Now, using this method, we can set the phase of the signal to 0 or π. f=(1 / 2)F NYIn this case, the phase of the signal can be set to 0, π / 2, π, 3π / 2. The second strategy for shifting the phase is to drive the digital-to-analog converter with different samplings of a given frequency. The waveforms of the first and second modulators are both designed such that the comb line transmission filter operates in resonance with the cavity round-trip time, similar to the operation of Fourier domain mode locking.

[0022] <Phase code filter performance> Before constructing the prototype PCML laser, the performance of the phase code filter was first characterized alone, i.e., outside the integration into the laser cavity. This was done using the setup shown in Fig. 1(b). Amplified spontaneous emission (ASE) light was used as the input from the SOA. This light passed through an 80 GHz Fabry-Perot etalon with a finesse of approximately 100. A second identical etalon was placed at the output and its angle was adjusted to align its comb line with that of the first etalon. An optical spectrum analyzer (Yokogawa, AQ6370C) was used to characterize the linewidth of each comb line as they passed through the filter. Due to the resolution limitation of the OSA, linewidths less than 0.02 nm could not be resolved. In the measurement shown in Fig. 3, the second modulator waveform was configured to decode the comb line at 1560 nm. It should be noted that all input comb lines were equally broadened by the first phase modulator (Fig. 3(b)), but only the 1560 nm comb line was significantly narrowed after the second phase modulator (Fig. 3(c)). Next, the transmission efficiency of each comb line was measured using the OSA. As expected, the narrowed comb line transmitted with high efficiency (Fig. 3(h)). The filter extinction of the other lines exceeded 3.5 dB (Fig. 3(h)).

[0023] <PCML laser performance> The filter was then assembled into a ring cavity as shown in Figure 2 and operated at three speeds, specifically 176 kHz (the 5th harmonic of the cavity), 881 kHz (the 25th harmonic), and 3.52 MHz (the 100th harmonic). These speeds were chosen to recover multiple output wavelength bands at frequencies that were integer multiples of the optical resonator's roundtrip frequency.

[0024] Figure 4 shows the lasing spectra and time traces of the PCML laser in 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 of these speeds, the laser output spanned 80 nm with a comb line spacing of 80 GHz. In this prototype laser using an 8 GSPS AWG, consistent performance was not achieved for pulse widths below 8.6 ns. To achieve a speed of 3.52 MHz while maintaining the 8.6 ns pulse width, the number of output wavelengths was reduced by designing the waveform to transmit every fourth comb line. It should be noted that these modifications to the laser performance only required changes to the AWG drive signal provided to the phase modulator and no hardware changes were required. This enabled the creation of a widely tunable and rapidly reconfigurable CR-OCT source.

[0025] While the worst-case single-pass extinction of the filter was about 3.5 dB, the laser output achieves much higher comb-line extinction due to the laser cavity resonance. The laser output spectrum shown in Figure 4(c) suggests a spectral extinction of over 25 dB, considering that the duty cycle of the selected lines is much lower than the off-lines. This can result in high instantaneous extinction of the laser.

[0026] The PCML laser coherence length was measured by acquiring an interference fringe signal as a function of the sample arm mirror position (with respect to the reference arm). The measurements were performed in configurations of 176 kHz and 881 kHz to analyze the relationship between the pulse width of the light source and the coherence length. It was calculated for the interference fringe signal at the same circular delay position at the order where the point spread function becomes large. The axial resolution was measured to be 17 μm from the acquired point spread function, which was consistent regardless of the presence or absence of the booster SOA. The coherence length (6 dB roll-off) was 85 mm (mirror displacement) for pulse widths of 43 ns (176 kHz A-line) and 8.6 ns (881 kHz A-line) (Figs. 5(a), (c)). This enables an imaging range of 170 mm using circular ranging that uses in-phase and quadrature interference fringe signal detection to distinguish between positive and negative delay spaces. It should be noted that the coherence length was measured to be the same for both pulse widths. Although not clearly measured, it is not expected that the coherence length would be affected by changing the output comb line sequence with the same pulse width (e.g., 881 kHz and 3.52 MHz output). Since the laser output power of the prototype was low (about 1 mW), the PCML coherence length was also tested with the booster SOA (outside the cavity). The booster SOA increased the power to 50 mW and decreased the coherence length to 50 mm (100 mm CR-OCT imaging range) as shown in Figs. 5(b), (d). This decrease is a result of the linewidth broadening in the SOA. It was also observed that the booster SOA increased the noise floor, which requires further investigation.

[0027] <CR-OCT Imaging Using the PCML Laser> CR-OCT images were acquired at each of the three imaging speeds (176 kHz, 881 kHz, and 3.52 MHz). As expected, the circular depth / delay range of the 3.52 MHz image was different from the 176 kHz and 881 kHz cases due to the generation of a 320 GHz frequency comb instead of an 80 GHz frequency comb. The laser was used with a booster SOA in the imaging experiments. A quadrature demodulation circuit as described in Siddiqui et al. ("Compensation of Spectral and rf Errors in Swept-Source OCT for High Extinction Complex Demodulation," Optics Express 23(5), 5508-5520 (2015), incorporated herein by reference) was used to generate the in-phase and quadrature interference fringes required for CR-OCT. The output interference fringes were sent to a balanced photoreceiver (Thorlabs, PDB465C) for detection. A digitizer (Signatec, PX14400) acquired signals from the detector at 250 MS / s, which is significantly higher than what is needed to capture the 8.6 ns (3.52 MHz and 881 kHz) and 43 ns (176 kHz) pulse widths. Figure 6 shows the acquired CR-OCT images of the IR detector card (Figure 6(a)-(c)) and finger (Figure 6(d)-(e)) at each lasing speed, which are averages of 25 adjacent sections. The circular range depth was approximately 1.9 mm for the 176 kHz and 881 kHz configurations and 0.48 mm for the 3.52 MHz configuration. It should be noted that the total imaging depth range and the circular delay range are separate parameters, and the signal can be acquired over the entire imaging range limited by the source coherence, but appears in a compressed image equal to the circular delay range.

[0028] We have presented an embodiment of a novel laser architecture 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. Operation of the laser at 3.52 MHz demonstrated that the PCML laser allows the user to set specific comb lines, skip comb lines, or otherwise modify the sequence of comb lines generated. This is done by designing the drive waveforms without hardware modifications and can be used to create highly reconfigurable sources for CR-OCT. The speed of the PCML laser is not directly defined by the magnitude of the intracavity dispersion as is the case for SPML lasers. This allows the PCML laser to be operated at more moderate speeds. At present, there is no clearly defined upper limit to the speed of the PCML laser. In principle, high speed operation is possible. However, it can be difficult to generate the high bandwidth RF drive signals that would be required, and at present, the SPML architecture is better suited for very high speed operation. Other advantages of the PCML architecture include 100% duty cycle output and linear-in-time output that does not require post-digital k-clocking or k-space resampling. Conversely, currently, the main drawback of PCML lasers is their noise performance, which is significantly higher than mature OCT laser technologies.

[0029] <Reducing relative intensity noise (RIN)> Certain embodiments of the present invention provide procedures for reducing noise in the disclosed sources. In certain embodiments, the disclosed PCML sources may impose high relative intensity noise (RIN) that may limit their usefulness in practice. Thus, procedures are disclosed herein that can tailor the driving waveforms of the phase modulators (EOMs) to generate discrete pulses (with "laser off" time between pulses) at each wavelength with a specific pulse duration (pulse width) related to the etalon finesse used in the cavity, making the PCML very stable (Figure 7). An empirical analysis of the laser noise for various waveform designs is presented herein (Figure 8). Results are also presented that show improved system performance (Figure 9) and imaging (Figure 10).

[0030] The noise reduction procedure can be performed using a system as disclosed above (see, for example, FIG. 8(a)). One modification of the system was to test several different etalons with similar FSR and different finesse for noise analysis, and one of the etalons was selected for the final laser demonstration.

[0031] The driving waveform of the phase modulator was designed to include an “on” and an “off” state for each pulse (Fig. 7(a)). The idea of ​​having an on and off state comes from the integration time (switch time) of the etalon, which is the range of group delay transit times for light of different optical frequencies to pass through the etalon. This integration time is the inverse of the linewidth of each etalon comb line. Light pulses of duration shorter than the integration time are stretched in time when passing through the etalon, resulting in pulses of duration roughly given by the integration time. By designing an off time of sufficient duration located between the pulses, it is possible to avoid pulses that overlap in time at the output of the etalon. Pulses that overlap in time would cause lasing instability and higher intensity noise. Based on this, it was determined that the optimal on / off time is a function of the etalon linewidth (which is a function of the etalon finesse and FSR).

[0032] During the laser on period, the phase modulator was controlled using the chirp waveform disclosed above, except that to achieve stable lasing over multiple cavity roundtrips, For the laser off period, only one chirp waveform was used instead of multiple waveforms. A uniform (non-chirped) sinusoidal waveform with different RF frequencies was used at each phase modulator during the laser off period (Fig. 7(b)). Various experiments were performed to find the optimal laser on / off times for different linewidths of the etalon. An optical pulse was generated during the waveform associated with the on-time at the selected wavelength determined by the waveform characteristics described above. The waveform associated with the off-time suppresses the transmission of all comb lines and temporarily turns off the laser.

[0033] In the first set of experiments, three different physical etalons were used, all with FSR between 80-85 GHz, and with finesses of 150, 500, and 8000. The three physical etalons were used to create laser sources with four different effective finesses: 150, 300 (double pass of 150 etalon), 500, and 8000. In the first experiment, a single laser oscillation line was created at about 1560 nm, with the repeated lines well spaced from each other in time, each with a different "laser on" time. The laser RIN and FWHM pulse width were investigated to find the optimal PW for RIN.

[0034] In the second set of experiments, dual line lasing at approximately 1560 nm and 1559 nm repeated over time was performed, with different "laser off" times. The laser RIN was investigated with respect to off-time to find the minimum off-time required to obtain the optimal RIN level. After finding the optimum RIN, additional experiments were performed to demonstrate improved PCML operation with improved RIN sensitivity. To compare image quality, imaging of subject finger skin was performed using the same imaging optics as described above.

[0035] Experiments were first performed to determine the noise performance of the improved laser as a function of "laser on" time. These measurements were performed with the laser configured to repeatedly generate pulses at the same wavelength (1560 nm). The laser noise performance was characterized as a function of time for the laser, which determines the output laser pulse width. The noise was characterized for all four etalon effective finesse levels (150, 300, 500, 8000) and for pulse widths from 0.134 ns to 300 ns (see Figure 8(a)). The laser off time for each configuration was 0.284 ns for finesse levels of 150, 300, and 500, and 0.568 ns for finesse level of 8000.

[0036] For all configurations tested, no lasing was observed when the "laser on" time was too short (Figure 8(a)). When the "laser on" time was long enough to achieve lasing, lasing began and tended to be fairly stable. With longer "laser on" times and larger pulse widths, the laser became unstable and the intensity noise increased significantly.

[0037] The observed pattern is that there is no lasing transition from stable to unstable lasing as the laser on-time / pulse width increases, and the scale factor of these transitions depends on the etalon finesse. This defines a strategy to optimally set the laser on-time for stable lasing depending on the etalon finesse. The optimal operating condition of the laser on-time is roughly given by the inverse of the linewidth of each etalon comb line, where the etalon linewidth is the etalon FSR divided by the finesse. Therefore, the optimal pulse width is roughly given by the etalon finesse divided by the etalon FSR.

[0038] Next, experiments were performed to determine the noise performance as a function of the "laser off" time. These measurements were performed with the laser configured to generate repetitive pulses alternating between two wavelengths (1560 nm and 1559 nm) as described above. See Figure 8 (b), (c), (d) and (e). The laser was configured to have a "laser off" timescale ranging from 0.134 ns to about 20 ns. The "laser on" times for each configuration were chosen from previous results to obtain stable lasing (1.88 ns for a finesse of 150, 2.97 ns for a finesse of 300, 5.38 ns for a finesse of 500 and 80.6 ns for a finesse of 8000).

[0039] Even though the "laser on" time is within the stable range, when the "laser off" time is too short, both pulses show a significant amount of noise, which means that they are not well separated. When the "laser off" time is longer than a certain duration, the pulses become stable, similar to the previous experiment. The "laser off" time threshold for stable lasing also corresponds to the etalon integration time, which is determined by the linewidth. Also, the RIN was found to be more consistent at higher finesse, which is probably due to the longer "laser on" time, which allows the signal from the AWG to have a more accurate and predictable quenching.

[0040] Finally, an experiment to characterize the noise of the PCML laser produced a full frequency comb output at 110 wavelengths. The PCML-OCT laser was demonstrated with an etalon having a finesse of 500 with an A-line rate of 1.16 MHz. The laser on-time and off-time were set to 3.6 ns and 4.4 ns, respectively, for each of the 110 pulses spanning a range of 75 nm centered at 1050 nm. This setting was derived from the results in Figure 8.

[0041] The ortho-RIN of each pulse was less than 1% as measured with a 2 GHz bandwidth detector and digitizer. The area of ​​the beam was the same as that of the source described above. The achieved system sensitivity was 104 dB at a sample arm power of 35 mW, with a 6 dB roll-off depth of 74 mm. From the work disclosed above, it can be assumed that the sensitivity roll-off of the PCML would be even longer without the booster SOA. Finger skin imaging also showed a significant SNR increase of about 20 dB compared to the previous work.

[0042] A particular etalon has an optimal pulse width, so if the pulse width is extended beyond this optimum to reduce imaging speed, the noise performance of the laser degrades. In one embodiment, the laser produces effective long pulses at a particular wavelength by repeating the same wavelength multiple times. The laser can be configured for slower speeds by generating a 50 ns pulse at 1550.00 nm. In one example, a 5 ns pulse at 1550.00 nm can be repeated 10 times, creating a pulse train of 50 ns duration containing 10 individual pulses. The detection system can convert this pulse train to 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 pulses repeated at each wavelength can be any integer, subject to the aforementioned constraint that the laser output sequence is equal to or a harmonic of the cavity round trip time.

[0043] <Computer and Optical Systems> Referring to FIG. 11, an example system (e.g., data acquisition and processing system) 1100 for providing a circular ranging OCT source is shown, according to some embodiments of the disclosed subject matter. In some embodiments, the computing device 1110 can execute at least a portion of the system 1104 for providing a CR-OCT source and provide control signals to one or more optical modulators associated with the optical cavity 1102. Additionally or alternatively, in some embodiments, the computing device 1110 can communicate information related to the control signals to and from the server 1120 via the communication network 1106, which can execute at least a portion of the system 1104 for providing a CR-OCT source. In some such embodiments, the server 1120 can return information related to the control signals of the system 1104 for providing a CR-OCT source to the computing device 1110 (and / or other suitable computing devices). This information may be transmitted and / or presented to a user (e.g., a researcher, an operator, a clinician, etc.) and / or may be stored (e.g., as part of a research database or medical record associated with the subject).

[0044] In some embodiments, the computing device 1110 and / or the server 1120 can be any suitable computing device or combination of devices, such as, for example, 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, the system 1104 for providing a source of CR-OCT can present information regarding the control signal to a user (e.g., a researcher and / or a physician). In some embodiments, the optical resonator 1102 can include optical components as disclosed herein (e.g., see FIG. 2(a)).

[0045] In some embodiments, the communications network 1106 may be any suitable communications network or combination of communications networks. For example, the communications 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. In some embodiments, the communications network 1106 may be a local area network, a wide area network, a public network (e.g., the Internet), a private or semi-private network (e.g., a corporate or university intranet), any other suitable type of network, or any suitable combination of networks. The communications links illustrated in FIG. 11 may each be any suitable communications link or combination of communications links, such as a wired link, an optical fiber link, a Wi-Fi link, a Bluetooth link, a cellular link, etc.

[0046] 12 illustrates example hardware 1200 that may be used to implement a computing device 1110 and a server 1120 in accordance with some embodiments of the disclosed subject matter. As illustrated in FIG. 12, in some embodiments, the computing device 1110 may include a processor 1202, a display 1204, one or more inputs 1206, one or more communication systems 1208, and / or a memory 1210. In some embodiments, the processor 1202 may be any suitable hardware processor or combination of hardware processors, such as a central processing unit, a graphics processing unit, etc. In some embodiments, the display 1204 may include any suitable display device, such as a computer monitor, a touch screen, a television, etc. In some embodiments, the input 1206 may include any suitable input device and / or sensor that may be used to receive user input, such as a keyboard, a mouse, a touch screen, a microphone, etc.

[0047] In some embodiments, communications system 1208 may include any suitable hardware, firmware, and / or software for communicating information over communications network 1106 and / or any other suitable communications network. For example, communications system 1208 may include one or more transceivers, one or more communications chips and / or chipsets, etc. In more specific examples, communications system 1208 may include hardware, firmware, and / or software that can be used to establish a Wi-Fi® connection, a Bluetooth® connection, a cellular connection, an Ethernet® connection, etc.

[0048] In some embodiments, memory 1210 may include any suitable storage device that may be used to store instructions, values, etc. that may be used by processor 1202, for example, to present content using display 1204, to communicate with server 1120 via communication system 1208, etc. Memory 1210 may include any suitable volatile memory, non-volatile memory, storage, or any suitable combination thereof. For example, 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, computer programs may be encoded in memory 1210 for controlling the operation of computing device 1110. In such embodiments, processor 1202 may execute at least a portion of the computer programs to present content (e.g., images, user interfaces, graphics, tables, etc.), receive content from server 1120, transmit information to server 1120, etc.

[0049] 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 hardware processors, such as a central processing unit or a graphics processing unit. 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 may be used to receive user input, such as a keyboard, a mouse, a touch screen, a microphone, etc.

[0050] In some embodiments, communications system 1218 may include any suitable hardware, firmware, and / or software for communicating information over communications network 1106 and / or any other suitable communications network. For example, communications system 1218 may include one or more transceivers, one or more communications chips, and / or chipsets. In a more specific example, the communications system 1218 may include hardware, firmware, and / or software that can be used to establish a Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, and the like.

[0051] In some embodiments, memory 1220 may include any suitable storage device or devices that may be used to store instructions, values, etc. that processor 1212 may use to present content using display 1214, communicate with one or more computing devices 1110, etc. Memory 1220 may include any suitable volatile memory, non-volatile memory, storage, or any suitable combination thereof. For example, 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, server programs may be encoded in memory 1220 to control the operation of server 1120. In such embodiments, processor 1212 may execute at least a portion of the server programs to transmit information and / or content (e.g., organization identification and / or classification results, user interfaces, 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.

[0052] 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 transitory or non-transitory. For example, non-transitory computer-readable medium may include media such as magnetic media (hard disks, floppy disks, etc.), optical media (compact disks, digital video disks, Blu-ray disks, etc.), semiconductor media (RAM, flash memory, electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), etc.), any suitable medium that is not transitory or lacks any semblance of permanence during transmission, and / or any suitable tangible medium. As another example, a transitory computer-readable medium may include signals on a network, in a wire, in a conductor, in an optical fiber, in a circuit, or any suitable medium that is transitory and lacks any semblance of permanence during transmission, and / or any suitable intangible medium.

[0053] It should be noted that the term mechanism, as used herein, can encompass hardware, software, firmware, or any suitable combination thereof.

[0054] FIG. 13 is a diagram of an interferometer system that can be used in conjunction with various embodiments of the present invention. FIG. 13 shows a Mach-Zehnder type interferometer that can be implemented using free space optics (FIG. 13A) or a fiber arrangement (FIG. 13B). Other interferometer types (e.g., Michelson, etc.) can also be applied. The light source LS in FIG. 13A or B can be an embodiment of a PCML laser as disclosed herein. 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 to the sample S. Backscattered light from the object of interest is then directed to the interferometer output (B11). In the reference arm, beam B12 is directed to a phase modulator (PM). The beam after PM (i.e., beam B13) is directed to the output of the interferometer, where it is combined with beam B11 by BS4 before being combined with beam B11. The output beam B14 is then detected by a detector D (e.g., a photodiode). Alternatively, a fiber-based interferometer, as shown in FIG. 13B, facilitates balanced detection with a phase shift of π between the output beams B14 and B15. The detected signals are then sampled at a sampling rate of f using a data collection and processing system, which may include a data acquisition board or a real-time oscilloscope (DAQ). S Multiple wavelength sweeps (A1, A2, ..., An) may be acquired to form a two- or three-dimensional image.

[0055] Thus, although the present invention has been described with reference to particular embodiments and examples, the present invention is not necessarily limited thereto, and many other embodiments, examples, uses, modifications, and evolutions of modifications from the embodiments, examples, uses are intended to be encompassed within the scope of the appended claims.

Claims

1. 1. A source for providing electromagnetic radiation within a particular spectral range, comprising: a ring-shaped optical resonator for circulating a plurality of wavelength bands, the optical resonator including a first optical phase modulator, a first wavelength dispersion device, a second optical phase modulator, a multi-line spectral domain filter, a second wavelength dispersion device, and an optical amplifier; a controller connected 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 first wavelength dispersion device is disposed between the first optical phase modulator and the second optical phase modulator and provides wavelength dispersion such that each of the plurality of wavelength bands experiences a respective plurality of different time delays; the first optical phase modulator and the second optical phase modulator are configured to modulate a first phase using the first optical phase modulator driven by the first waveform, and modulate the second phase after a specific time delay using the second optical phase modulator driven by the second waveform comprising an inverse waveform of the first waveform, thereby producing spectral broadening by the first optical phase modulator for each of the plurality of wavelength bands and spectral restoration by the second optical phase modulator for a specific wavelength band of the plurality of wavelength bands; the particular time delay is determined to produce a spectral reconstruction for the particular wavelength band of the plurality of wavelength bands; the multi-line spectral domain filter is configured to provide multi-line spectral filtering with narrow bandwidths to cause power loss in each of the plurality of wavelength bands other than the particular wavelength band; the second chromatic dispersion device is configured to perform chromatic dispersion compensation on the output of the multi-line spectral domain filter to compensate for group delay dispersion within the optical cavity to match a roundtrip frequency for each of the plurality of wavelength bands; The first waveform and the second waveform are configured to produce a periodic phase modulation to restore the multiple wavelength bands at a frequency that is an integer multiple of a rotation frequency of the optical resonator.

2. The source of claim 1 , wherein the optical cavity further comprises an optical isolator configured to provide optical isolation for unidirectional transmission of laser light in the optical cavity.

3. 3. The source of claim 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. 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. 3. The source of claim 1 or 2, wherein the first wavelength dispersive device is a dispersive fiber, a chirped fiber Bragg grating, a fiber Bragg grating array, or a reflective fiber delay line.

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

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

8. 3. The source of claim 1 or 2, wherein the multi-line spectral domain filter comprises a Fabry-Perot etalon.

9. 3. A source according to claim 1 or 2, wherein the optical amplifier is a semiconductor optical amplifier or an erbium-doped fibre amplifier.

10. 3. The source of claim 1 or 2, wherein the first chromatic dispersion device provides anomalous chromatic dispersion and the second chromatic dispersion device provides normal chromatic dispersion.

11. 3. The source of claim 1 or 2, wherein the first chromatic dispersion device provides normal chromatic dispersion and the second chromatic dispersion device provides anomalous chromatic dispersion.

12. 3. The source of claim 1 or 2, wherein the controller comprises a two-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. 13. The source of claim 12, wherein the first waveform and the second waveform are generated by different discrete representations of the first waveform and the second waveform such that a delay between the first waveform and the second waveform can be controlled with a precision greater than a digital-to-analog sampling frequency.

15. 3. The source of claim 1 or 2, wherein the optical resonator further comprises an output coupler configured to emit a first output electromagnetic radiation based on the multiple wavelength bands.

16. 3. The source of claim 1 or 2, wherein the specific time delays are adjusted such that each of the plurality of wavelength bands is spectrally restored in wavelength order by the second optical phase modulator to generate a wavelength-stepped laser.

17. 3. The source of claim 1 or 2, wherein at least one of the first waveform or the second waveform comprises a chirped sinusoidal waveform.

18. 3. The source of claim 1 or 2, wherein the first waveform and the second waveform are configured to produce a periodic phase modulation to recover the multiple wavelength bands at a frequency between 100 kHz and 5 MHz.

19. 3. The source of claim 1 or 2, wherein the source is operated with laser-on and laser-off periods.

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

21. 3. A source according to claim 1 or 2, wherein the source produces optical pulses having a duration (pulse width) related to the inverse of the linewidth of the Fabry-Perot transmission peak.

22. 1. A source for providing electromagnetic radiation within a particular spectral range, comprising: A ring-shaped optical resonator for circulating a plurality of wavelength bands, comprising a first optical phase modulator and an optical resonator including a wavelength dispersion device, a second optical phase modulator, a multi-line spectral domain filter, and an optical amplifier; a controller connected 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 wavelength dispersion device is disposed between the first optical phase modulator and the second optical phase modulator, and provides wavelength dispersion such that each of the plurality of wavelength bands experiences a respective plurality of different time delays; the first optical phase modulator and the second optical phase modulator are configured to modulate a first phase using the first optical phase modulator driven by the first waveform, and modulate the second phase after a specific time delay using the second optical phase modulator driven by a second waveform comprising an inverse waveform of the first waveform, thereby producing spectral broadening by the first optical phase modulator for each of the plurality of wavelength bands and spectral restoration by the second optical phase modulator for a specific wavelength band of the plurality of wavelength bands; the particular time delay is determined to produce a spectral reconstruction for the particular wavelength band of the plurality of wavelength bands; the multi-line spectral domain filter is configured to provide multi-line spectral filtering with narrow bandwidths to cause power loss in each of the plurality of wavelength bands other than the particular wavelength band; the first waveform and the second waveform are configured to produce a restoration having a duration sufficiently long so that each of the plurality of wavelength bands makes multiple revolutions within the optical cavity; The first waveform and the second waveform are configured to produce a periodic phase modulation to restore the multiple wavelength bands at a frequency that is an integer multiple of a rotation frequency of the optical resonator.

23. 23. The source of claim 22, wherein the optical cavity further comprises an optical isolator configured to provide optical isolation for unidirectional transmission of laser light in the optical cavity.

24. 24. The source of claim 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. 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. 24. A source according to claim 22 or 23, wherein the wavelength dispersive device is a dispersive fibre, a chirped fibre Bragg grating, a fibre Bragg grating array or a reflective fibre delay line.

27. 24. The source of claim 22 or 23, wherein the multi-line spectral domain filter comprises a Fabry-Perot etalon.

28. 24. A source according to claim 22 or 23, wherein the optical amplifier is a semiconductor optical amplifier or an erbium doped fibre amplifier.

29. 24. A source according to claim 22 or 23, wherein the controller comprises a two-channel arbitrary waveform generator.

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

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

32. 24. The source of claim 22 or 23, wherein the optical resonator further comprises an output coupler configured to emit a first output electromagnetic radiation based on the multiple wavelength bands.

33. 24. The source of claim 22 or 23, wherein the specific time delays are adjusted such that each of the plurality of wavelength bands is spectrally restored in wavelength order by the second optical phase modulator to generate a wavelength-stepped laser.

34. 24. The source of claim 22 or 23, wherein at least one of the first waveform or the second waveform comprises a chirped sinusoidal waveform.

35. 24. A source according to claim 22 or 23, wherein the first waveform and the second waveform are configured to produce a periodic phase modulation to recover the multiple wavelength bands at a frequency between 100 kHz and 5 MHz.

36. 24. A source according to claim 22 or 23, wherein the source operates with laser-on and laser-off periods.

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

38. 24. A source according to claim 22 or 23, wherein the source produces optical pulses having a duration (pulse width) related to the inverse of the linewidth of the Fabry-Perot transmission peak.

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