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 Long-range optical coherence tomography (CR-OCT) utilizes a compressed echo delay ranging approach to reduce the number of measurements required to capture signals 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 for 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 are not easily scalable to lower speeds and therefore do not offer a solution for medium-speed CR-OCT. Instead, alternative source technologies are 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 includes 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, wherein 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 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 then modulate a second phase using a second optical phase modulator driven by a second waveform, the second waveform being an inverse of the first waveform, after a specific time delay, to cause 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 specific wavelength band of the plurality of wavelength bands, the specific time delay being determined to cause spectral restoration for a specific 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 power loss in each of a plurality of wavelength bands other than the particular wavelength band, and the second wavelength dispersion device is configured to perform wavelength dispersion compensation on the output of the multi-line spectral domain filter to compensate for group delay dispersion in the optical resonator and match roundtrip frequencies for each of the plurality of wavelength bands, and the first waveform and the second waveform is configured to produce periodic phase modulation for recovering multiple wavelength bands at frequencies that are integer multiples of the roundtrip frequency of the optical resonator.

[0006] In another embodiment, the present invention provides a source for providing electromagnetic radiation within a specific 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 experiences a respective plurality of different time delays, the first optical phase modulator and the second optical phase modulator modulating the first phase using the first optical phase modulator driven by the first waveform, and after a specific time delay, driving a second waveform comprising an inverse waveform of the first waveform. the first waveform and the second waveform are configured to produce periodic phase modulation for restoring the plurality of wavelength bands at a frequency that is an integer multiple of the rotation 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 numerals refer to like elements and in which: [Brief explanation of the drawings]

[0008] [Figure 1] This figure shows the working principle of the phase-encoded wavelength filter. (a) shows the linewidth expansion 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 the phase modulator, FG is the function generator, and SOA is the semiconductor optical amplifier. [Figure 2] (a) shows the PCML laser setup, 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. [Figure 3] Single-pass phase-encode filter performance measurements are shown for each comb line. 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 wavelength, tp=43 ns, (b) 881 kHz, 130 wavelength, tp=8.6 ns, and (c) 3.52 MHz, 31 wavelength, tp=8.6 ns. [Figure 5] The 6 dB roll-off measurements of the PCML laser are shown for various settings: (a) 176 kHz without the booster SOA, (b) 176 kHz with the booster SOA, (c) 881 kHz without the booster SOA, and (d) 881 kHz with the booster SOA. [Figure 6](a)-(c) show cross-sectional images of an IR card, and (d) and (e) show cross-sectional images of a finger with different A-line configurations at the same scale and corresponding circular area. Data were collected at three different imaging rates: (a) and (d) 176 kHz, (b) and (e) 881 kHz, and (c) 3.52 MHz. The dynamic range was (a) and (b) 20 dB, (c) 10 dB, and (d) and (e) 18 dB. Scale bar (bottom 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 plots for a single-pulse experiment, where the red, blue, green, and purple dots correspond to 150, 300, 500, and 8000 finesse at 1560 nm, respectively. (b)–(e) show the orthogonal RIN plots for a double-pulse experiment for 150, 300, 500, and 8000 finesse, respectively. The blue dots correspond to the first 1560 nm pulse, and the orange dots correspond to the second 1559 nm pulse. [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 and 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 INVENTION

[0009] According to 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-code mode-locked (PCML) laser architecture that can operate in the kilohertz to megahertz range and also offers 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 demonstrated with a coherence-length-limited imaging depth of 170 mm at A-line rates of 176 kHz to 3.52 MHz. 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 fast stepped frequency comb sources. Compressed ranging requires fewer measurements 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 exceeding 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 to medium-speed imaging, requiring very long chirped fiber Bragg gratings 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 produces the required stepped-frequency comb output, but its noise and coherence length characteristics are relatively poor, and the temporally 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-code mode-locking (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, while PCML architectures use electro-optic phase modulation. This difference, combined with the use of appropriately designed control signals, allows the laser repetition rate to be decoupled from the intracavity dispersion. As a result, the PCML laser embodiments disclosed herein are capable of operating at moderate speeds using moderate levels of intracavity dispersion. Furthermore, PCML sources can operate over 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 utilize 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, a narrowband CW input field variation is first analyzed at optical frequency ω, passing through two successive phase modulators (Figure 1(a)) to produce a frequency comb line transmission filter. A (t) = cos(ωt). Light is emitted at point A. The first modulator is driven by a voltage drive signal f1(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 (expands) 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 The signal is further modulated as f(t) = cos(ω(t-dn / c) + αf(t-dn / c)) + f(t)). If the drive signal is configured such that f(t) = -f(t-dn / c), the two modulations cancel, restoring the original narrowband linewidth and resulting in E D It is easy to see that (t) = cos(ωt). Note that this reversible linewidth broadening occurs only if the delay of signal f2 relative to f1 matches the optical group delay between the modulators.

[0016] In Figure 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 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. Here, 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)). An output Fabry-Perot etalon (identical to the first etalon) then attenuates the optical power of the broad comb lines while efficiently transmitting a single narrow comb line. By controlling the drive signal provided to the second modulator, any comb line can be selected for high transmission. Extending this concept, a dynamic sequence of comb line transmissions can be generated by applying a properly 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-encoded filter shown in Figure 1 was placed in a ring cavity (Figure 2(a)). Within this cavity, light travels 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 with 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-encoded filter contained two lithium niobate phase modulators (Covega) with 10 GHz RF bandwidth. An arbitrary waveform generator (Euvis, AWG872) provided the drive signal to these modulators via RF amplifiers. A dispersion-compensating fiber (OFS, WBDK:84C-L) provided dispersion of -84 ps / nm at 1550 nm between the phase modulators. To balance the cavity round-trip time across wavelengths, For equalization (dispersion matching), a Faraday rotator mirror (FRM) was used to insert approximately 2.4 km of SMF-28e+ fiber into the cavity in a double-pass configuration. 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 drive waveforms provided to the first and second phase modulators set the filter transmission characteristics over time. 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 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. In the embodiments shown herein, a chirp sine wave The reason for using a chirped sinusoidal wave is that it lacks a dominant frequency component, which would cause periodicity in the delay response (i.e., an autocorrelation function), resulting in multiple comb lines potentially being transmitted through the filter.

[0019] For the first modulator, a repeating waveform was constructed to be provided to the first modulator (f1(t)). This waveform was constructed by combining sinusoidal 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] Next, the waveform provided to the second modulator was constructed by combining sections of the chirped sinusoids used to form the first waveform. These chirped sinusoids 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. Next, 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 described above. The chirped sinusoidal waveforms were then 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 approximately 54 ps. Because this is significantly smaller than the clock cycle of the arbitrary waveform generator (125 ps), we induced nearly arbitrary delays in the waveform by generating appropriately different discrete representations of the chirped waveform and sending these new discrete representations 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 create a delay with sub-clock cycle accuracy. For example, a digital-to-analog converter with sampling frequency F can be generated at the Nyquist frequency F. NY It is known that signals can be generated with various phases up to πn(f / F). There are two strategies for shifting 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 at 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 resonantly 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 of integration into the laser cavity. This was done using the setup shown in (b) of Fig. 1. 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 ((b) of Fig. 3), but only the 1560 nm comb line was significantly narrowed after the second phase modulator ((c) of Fig. 3). Next, the transmission efficiency of each comb line was measured using the OSA. As expected, the narrowed comb line transmitted with high efficiency ((h) of Fig. 3). The filter extinction of the other lines exceeded 3.5 dB ((h) of Fig. 3). It should be noted that all input comb lines were equally broadened by the first phase modulator ((b) of Fig. 3), but only the 1560 nm comb line was significantly narrowed after the second phase modulator ((c) of Fig. 3). Next, the transmission efficiency of each comb line was measured using the OSA. As expected, the narrowed comb line transmitted with high efficiency ((h) of Fig. 3). The filter extinction of the other lines exceeded 3.5 dB ((h) of Fig. 3).

[0023] <PCML laser performance> The filter was then assembled into a ring cavity as shown in Figure 2 and operated at three speeds: 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 are 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 43 ns pulse width was used. At 881 kHz, an 8.6 ns pulse width was used. For both of these speeds, the laser output spanned 80 nm with an 80 GHz comb line spacing. In this prototype laser using an 8 GSPS AWG, consistent performance was not achieved at pulse widths less than 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. Note that these modifications to the laser performance required only changes to the AWG drive signal provided to the phase modulator; 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 obtaining 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 obtained 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) with 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 employs 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 sequence of the output comb lines with the same pulse width (e.g., 881 kHz and 3.52 MHz outputs). Since the prototype laser output power 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 broadening of the linewidth 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 three imaging speeds: 176 kHz, 881 kHz, and 3.52 MHz. As expected, the circular depth / delay range of the 3.52 MHz image differed from that of the 176 kHz and 881 kHz cases due to the generation of a 320 GHz frequency comb rather than an 80 GHz frequency comb. The laser was used with a booster SOA in the imaging experiments. The quadrature demodulation circuit 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 that required to capture pulse widths of 8.6 ns (3.52 MHz and 881 kHz) and 43 ns (176 kHz). Figure 6 shows the acquired CR-OCT images of the IR detector card (Figures 6(a)-(c)) and finger (Figures 6(d)-(e)) at each lasing speed, which are averages of 25 adjacent sections. The circular depth range 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; signals can be acquired over the entire imaging range, limited by source coherence, but appear in a compressed image equal to the circular delay range.

[0028] We present 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 offers several unique features. Operation of the laser at 3.52 MHz demonstrates that the PCML laser allows the user to configure specific comb lines, skip comb lines, or change the sequence of generated comb lines. This is achieved by designing the drive waveform without hardware modifications and can be used to create a highly reconfigurable source for CR-OCT. The speed of PCML lasers is not directly defined by the magnitude of the intracavity dispersion, as is the case with SPML lasers. This allows PCML lasers to operate at more moderate speeds. At present, the upper speed limit for PCML lasers has not been clearly defined. Higher speeds are possible in principle. However, generating the high-bandwidth RF drive signals likely required can be challenging, and the SPML architecture is currently more suitable 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, the main drawback of PCML lasers at present is their noise performance, which is significantly higher than that of 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 can impose high relative intensity noise (RIN), which can limit their usefulness in practice. Accordingly, procedures are disclosed herein that can make PCML highly stable by tailoring the drive waveform of the phase modulator (EOM) to generate discrete pulses (with "laser off" time between pulses) at each wavelength with specific pulse durations (pulse widths) related to the etalon finesse used in the cavity (FIG. 7). Empirical analysis of laser noise for various waveform designs is presented herein (FIG. 8). Results are also presented that demonstrate improved system performance (FIG. 9) and imaging (FIG. 10).

[0030] The noise reduction procedure can be performed using a system such as that 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 final laser demonstration.

[0031] The driving waveform of the phase modulator was designed to include an "on" and an "off" state for each pulse (Figure 7(a)). The idea of ​​having on and off states comes from the etalon's integration time (switch time), which is the range of group delay transit times for light of different optical frequencies passing through the etalon. This integration time is the inverse of the linewidth of each etalon comb line. Optical pulses with a duration shorter than the integration time are stretched in time as they pass through the etalon, resulting in pulses with a duration roughly given by the integration time. By designing an off time of sufficient duration located between pulses, overlapping pulses at the etalon output can be avoided. Overlapping pulses cause lasing instability and higher intensity noise. Based on this, the optimal on / off times were determined to be 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 this purpose, only one chirped waveform was used instead of multiple waveforms. During the laser-off period, a uniform (unchirped) sinusoidal waveform with a different RF frequency was used for each phase modulator (Fig. 7(b)). Various experiments were conducted to find the optimal laser on / off times for various etalon linewidths. An optical pulse was generated during the waveform associated with the on-time at a selected wavelength determined by the waveform characteristics described above. The waveform associated with the off-time suppressed the transmission of all comb lines and temporarily turned the laser off.

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

[0034] In the second set of experiments, dual-line laser oscillations at approximately 1560 nm and 1559 nm were performed, repeating over time, with different "laser off" times. The laser RIN was investigated in relation to the 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, the same imaging optics as described above were used to image the skin of a subject's finger.

[0035] Experiments were first conducted 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, and 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 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. As the "laser on" time was further increased and the pulse width was increased, 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 for optimally setting the laser on-time for stable lasing depending on the etalon finesse. The optimal operating condition for 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 conducted to determine noise performance as a function of "laser-off" time. These measurements were performed with a laser configured to repeatedly generate 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 approximately 20 ns. The "laser-on" times for each configuration were chosen from previous results to ensure 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 was within the stable range, when the "laser-off" time was too short, both pulses exhibited a significant amount of noise, meaning they were not sufficiently separated. When the "laser-off" time was longer than a predetermined duration, the pulses became stable, as in the previous experiment. The "laser-off" time threshold for stable lasing also corresponds to the etalon integration time, which is determined by the linewidth. We also found that the RIN was more consistent at higher finesse, likely due to the longer "laser-on" time, which allows the signal from the AWG to have more precise and predictable quenching.

[0040] Finally, experiments to characterize the noise of the PCML laser generated a full frequency comb output at 110 wavelengths. The PCML-OCT laser was demonstrated with a finesse etalon of 500 with an A-line rate of 1.16 MHz. For each of the 110 pulses spanning a 75 nm range centered at 1050 nm, the laser on-time and off-time were set to 3.6 ns and 4.4 ns, respectively. 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 axial section was the same as 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. Imaging of finger skin also showed a significant SNR increase of about 20 dB compared to previous work.

[0042] A particular etalon has an optimum pulse width, so if the pulse width is extended beyond this optimum to reduce imaging speed, the noise performance of the laser will degrade. In one embodiment, the laser generates 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 ten times, creating a pulse train of 10 individual pulses each 50 ns in duration. 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 repeated pulses at each wavelength can be any integer, subject to the aforementioned constraint that the laser output sequence be equal to or a harmonic of the cavity roundtrip time.

[0043] <Computer and Optical Systems> 11 , an example system (e.g., a data acquisition and processing system) 1100 for providing a circular ranging OCT source is shown, in accordance with some embodiments of the disclosed subject matter. In some embodiments, a computing device 1110 can execute at least a portion of a system for providing a CR-OCT source 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 communicate information related to the control signals to a server 1120 via a communications network 1106, which can execute at least a portion of the system for providing a CR-OCT source 1104. In some such embodiments, the server 1120 can return information related to the control signals of the system for providing a CR-OCT source 1104 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 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 CR-OCT source can present information regarding control signals 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, communications network 1106 may be any suitable communications network or combination of communications networks. For example, 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, 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 shown 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 can be used to implement a computing device 1110 and a server 1120 in accordance with some embodiments of the disclosed subject matter. As shown in FIG. 12 , in some embodiments, the computing device 1110 can 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, the processor 1202 can 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 can include any suitable display device, such as a computer monitor, a touchscreen, a television, etc. In some embodiments, the input 1206 can include any suitable input device and / or sensor that can be used to receive user input, such as a keyboard, a mouse, a touchscreen, 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 processor 1202 may use, 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, memory 1210 may be encoded with a computer program for controlling the operation of computing device 1110. In such embodiments, 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 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 touchscreen, 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 touchscreen, 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, etc.

[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, for example, to present content using display 1214, to 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, memory 1220 may be encoded with a server program for controlling the operation of server 1120. In such embodiments, processor 1212 may execute at least a portion of the server program to transmit information and / or content (e.g., tissue 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 discs, digital video discs, Blu-ray Discs, 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, 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 temporary medium that 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 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 13B 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). Beam 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 the PM (i.e., beam B13) is directed to the interferometer output, 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 signal is 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-dimensional or three-dimensional image.

[0055] Thus, although the present invention has been described in relation to particular embodiments and examples, the invention is not necessarily limited thereto, and many other embodiments, examples, uses, modifications, and departures from the embodiments, examples, and uses are intended to be included 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 a 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 causing 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 resonator and match a circulating 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 recover the plurality of wavelength bands at a frequency that is an integer multiple of a roundtrip frequency of the optical resonator; the first waveform comprises a waveform of a chirp sinusoidal signal; The second waveform includes an inverted and delayed coupling section of the chirped sinusoidal signal of the first waveform.

2. 10. The source of claim 1, further comprising an optical isolator configured to provide optical isolation for unidirectional transmission of output laser light of 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-optic 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, 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, wherein the multi-line spectral domain filter comprises a Fabry-Perot etalon.

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

10. 3. The source of claim 1, wherein the first wavelength dispersive device provides anomalous wavelength dispersion and the second wavelength dispersive device provides normal wavelength dispersion.

11. 3. The source of claim 1, wherein the first wavelength dispersive device provides normal wavelength dispersion and the second wavelength dispersive device provides anomalous wavelength 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 that amplifies 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 plurality of wavelength bands.

16. 3. The source of claim 1, wherein the specific time delay is 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, wherein the first waveform and the second waveform are configured to produce a periodic phase modulation for recovering the plurality of wavelength bands at a frequency between 100 kHz and 5 MHz.

19. 3. The radiation source of claim 1, wherein the radiation source operates with laser-on periods 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 generates 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, 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 second 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 then, after a specific time delay, modulate a second phase using the second optical phase modulator driven by a second waveform comprising an inverse waveform of the first waveform, thereby causing 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 round trips within the optical resonator; the first waveform and the second waveform are configured to produce a periodic phase modulation to recover the plurality of wavelength bands at a frequency that is an integer multiple of a roundtrip frequency of the optical resonator; the first waveform comprises a waveform of a chirp sinusoidal signal; the second waveform includes an inverted and delayed coupling section of the chirped sinusoidal signal of the first waveform; The particular time delay is 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.

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

24. 24. A source according to claim 22 or 23, wherein at least one of the first optical phase modulator and the second optical phase modulator comprises an electro-optic 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 first wavelength dispersive device is a dispersive fiber, a chirped fiber Bragg grating, a fiber Bragg grating array, or a reflective fiber 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 fiber 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 that amplifies 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 precision of the specified 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 plurality of wavelength bands.

33. 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.

34. 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 for recovering the plurality of wavelength bands at a frequency between 100 kHz and 5 MHz.

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

36. 36. The source of claim 35, 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.

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

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