Adjusting the laser output

Abstract

In a general aspect, the laser system includes a laser and a frequency comb generator system. The laser is configured to generate a laser signal, and the frequency comb generator system is configured to generate a frequency comb based on the laser signal. The frequency comb includes a frequency comb signal at a respective comb frequency. The laser system also includes a frequency comb distribution system configured to spatially separate the frequency comb signal into respective optical channels of the frequency comb distribution system. The laser system further includes a frequency selector system configured to generate a selected frequency signal from the frequency comb signal after separation. The selected frequency signal includes a target separated frequency comb signal. The laser system also includes a frequency shifter configured to shift the selected frequency signal toward a target output frequency of the laser system.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 18 / 150,110, entitled "Tuning the Output of a Laser," filed January 4, 2023, and U.S. Provisional Patent Application No. 63 / 324,740, also entitled "Tuning the Output of a Laser," filed March 29, 2022. The disclosures of the priority applications are incorporated herein by reference in their entireties.

[0002] Government License Rights This invention was made with U.S. Government support under Contract No. HR001121C0141, Subcontract No. 59604 to SRI International under prime contract with the Defense Advanced Research Projects Agency (DARPA) of the Department of Defense. The Government has certain rights in this invention.

[0003] The following description relates to adjusting the output of a laser. [Background technology]

[0004] Lasers can be used to output a beam of coherent light. A conventional laser includes a gain medium (e.g., a doped crystal, chemical compound, gas, etc.) and an external energy source (e.g., a flash lamp, another laser, etc.) to stimulate the gain medium. The gain medium generates photons, the intensity of which is amplified in response to stimulation from the external energy source. The generated photons have respective wavelengths that define the spectral bandwidth of the laser. [Brief description of the drawings]

[0005] [Figure 1] 1 is a schematic diagram of an exemplary laser system having adjustable output power. [Diagram 2] 1 is a schematic diagram of an exemplary external cavity diode laser. [Diagram 3]FIG. 1 is a schematic diagram of an exemplary frequency comb generated by a mode-locked laser. [Figure 4] FIG. 1 is a schematic diagram of an exemplary laser system including a continuous wave laser, a frequency comb, a frequency filter, and a fine-tuning element. [Diagram 5] FIG. 1 is a schematic diagram of an exemplary laser system including a frequency reference defined by a water vapor cell. [Figure 6] FIG. 1 is a diagram of an exemplary external cavity diode laser including a piezoelectric mounted mirror, a gain chip, and angled optics that function as an interference filter. [Figure 7] 7 is a schematic diagram of the example cavity diode laser of FIG. 6 illustrating the optical path extending from the piezoelectric mounted mirror through the adjustable laser gain chip and angled optics. [Figure 8] 1 is a graph of an exemplary frequency comb generated by one intensity modulator and one phase modulator. [Figure 9] FIG. 1 is a schematic diagram of a portion of an exemplary laser system including an 80-channel arrayed waveguide grating coupled to an optical switch. [Figure 10] FIG. 1 is a schematic diagram of an exemplary laser system including photonic integrated circuit elements for generating a frequency comb and filtering an optical signal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] In a general aspect, a laser system is described herein that can be agile and highly adjustable. In many implementations, the laser system is configured to generate laser light with a narrow spectral bandwidth (e.g., less than 10 MHz). The spectral bandwidth can be narrow enough to handle atomic and molecular beams in frequency domain spectroscopy, and in certain cases, can even be about 1 Hz. Such laser light can handle atomic transitions over a wide range of frequencies with fast switching speeds (e.g., 1 MHz spectral bandwidth in 1 microsecond). The laser system can include a frequency-stabilized continuous wave (CW) laser, a frequency comb, a frequency filter, an optical switch (or optical switch network), and a frequency shifter. In this configuration, the laser system can minimize the response time required for the output to switch between different precise target frequencies, thus enabling fast adjustment of the output.

[0007] Referring now to FIG. 1, a schematic diagram of an exemplary laser system 100 having a tunable output is depicted. The exemplary laser system 100 includes a laser 102 configured to generate a laser signal 104. The laser 102 may be a continuous wave (CW) laser in certain cases, and in these cases, the laser signal may be a frequency-locked output of the continuous wave laser. For example, as illustrated in FIG. 1, the exemplary laser system 100 may include a vapor cell 106 having a vapor therein. The frequency-locked output of the laser 102 may be based on an electronic transition of the vapor. As another example, the exemplary laser system 100 may include an optical cavity (e.g., an interferometer), and the frequency-locked output of the laser 102 may be based on an output frequency of the optical cavity as determined by an optical path length of the optical cavity. The output frequency may be associated with a reflected beam from the optical cavity. However, in certain cases, the output frequency is associated with a transmitted beam from the optical cavity.

[0008] The exemplary laser system 100 also includes a frequency comb generator system 108 configured to generate a frequency comb based on the laser signal 102. The frequency comb includes a frequency comb signal 110 (e.g., comb teeth) at a respective comb frequency. The exemplary laser system 100 further includes a frequency comb distribution system 112 configured to spatially separate the frequency comb signal 110 into respective optical channels 114 of the frequency comb distribution system 112. The optical channels 114 may be spatially separated from one another (e.g., physically separated from one another by a fixed distance). In some variations as illustrated in FIG. 1, the frequency comb distribution system 112 includes an arrayed waveguide grating (AWG). The arrayed waveguide grating has an input optical channel 114a that receives the frequency comb signal 110 from the frequency comb generator system 108. The arrayed waveguide grating may also have multiple output optical channels 114b that are physically separated from one another by a fixed distance.

[0009] The exemplary laser system 100 also includes a frequency selector system 116 configured to generate a selected frequency signal 118 from the frequency comb signal 110 after separation (e.g., after separation into respective optical channels 114 of the frequency comb distribution system 112). The selected frequency signal 118 includes a target separated frequency comb signal. In some cases, the target separated frequency comb signal has a comb frequency that is closest to the target output frequency of the laser system. However, other comb frequencies are possible. For example, the target separated frequency comb signal may have a comb frequency that is second closest to the target output frequency of the laser system, a comb frequency that is third closest to the target output frequency of the laser system, etc. The comb frequency may be determined by the maximum frequency shifting capability of the frequency shifters of the laser system 100 in certain cases, as described further below. In some variations, the frequency selector system 116 includes a network of optical switches (e.g., a series of optical switches). In these variations, the exemplary laser system 100 includes a control system configured to control the network of optical switches to select a target optical channel (e.g., of an arrayed waveguide grating) that carries the target separated frequency comb signal.

[0010] In some variations, the exemplary laser system 100 includes a photonic integrated circuit (PIC). The photonic integrated circuit may have a network of dropout filters and may define the frequency comb distribution system 112. In some variations, the photonic integrated circuit further defines the frequency selector system 116. The network of dropout filters may include, for example, a plurality of ring resonators associated with respective frequency comb signals of the frequency comb. Each ring resonator is configured to resonate at a ring frequency when the optical path length of the ring resonator reaches a target optical path length. The optical path length may be changed, for example, by straining the ring resonator via a piezoelectric element or by changing the temperature of the ring resonator via a heating element. Such a process may change one or both of the dimensions and the refractive index of the optical path length. The ring frequency of the ring resonator matches the comb frequency of one of the frequency comb signals. Thus, by selectively moving a particular ring resonator in and out of resonance, the photonic integrated circuit may enable the exemplary laser system 100 to select a comb frequency that is closest to the target output frequency of the exemplary laser system 100.

[0011] The exemplary laser system 100 also includes a frequency shifter 120 configured to modify the selected frequency signal 118 toward a target output frequency. For example, the frequency shifter 120 may be configured to modify the selected frequency signal 118 by either or both of fine tuning the selected frequency signal 118 to the target output frequency and scanning the selected frequency signal 118 around the target output frequency. The process of fine tuning the selected frequency signal 118 may include shifting the selected frequency signal 118 along a single direction in frequency toward the target output frequency. The process of scanning the selected frequency signal 118 may include shifting the selected frequency signal 118 back and forth across a frequency range that includes the target output frequency. The frequency range of the scanning process may be greater in magnitude than the frequency range associated with the fine tuning process.

[0012] In some variations, the frequency shifter 120 has a maximum shifting capability that determines which comb frequencies are possible for the target separated frequency comb signal. The maximum shifting capability may be based on the frequency range of the fine-tuning or scanning process. For example, the fine-tuning process may be associated with a frequency range that includes the target output frequency of the laser system therein. Thus, the comb frequencies may also include frequencies within the frequency range of the fine-tuning process. In some variations, the frequency shifter 120 includes an optical modulator that performs single-sideband suppressed carrier (SSB-SC) modulation. In some variations, the comb frequencies of the frequency comb signal 110 are separated from each other by a common frequency interval. In these variations, the frequency shifter 120 may be configured to change the selected frequency signal 118 by an amount of frequency equal to or greater than the common frequency interval.

[0013] During operation, the exemplary laser system 100 generates a frequency comb having frequency comb signals 118 at respective comb frequencies by operation of the frequency comb generator system 108. The frequency comb is based on the laser signal 104. The exemplary laser system 100 also spatially separates the frequency comb signals 110 into respective optical channels 114 of the frequency comb distribution system 112 by operation of the frequency comb distribution system 112. A selected frequency signal 118 may be generated from the separated frequency comb signal by operation of the frequency selector system 116. The selected frequency signal 118 includes a target separated frequency comb signal. The exemplary laser system 100 further shifts the selected frequency signal 118 toward a target output frequency by operation of the frequency shifter 120.

[0014] During operation, the target output frequency may be changed to a new target output frequency, and the example laser system 100 changes its output to match the new target output frequency. To do this, the example laser system 100 may include a control system. The control system may be configured, for example, to control the frequency selector system 116 (e.g., to control a network of optical switches). In this configuration, the control system may instruct the frequency selector system 116 to select the target optical channel 114 that carries the target isolated frequency comb signal (e.g., the frequency comb signal that is closest to the target output frequency of the example laser system 100). However, the control system may control other components of the example laser system 100 (e.g., the laser system 102, the frequency shifter 120, etc.).

[0015] In some variations, the control system is operable to communicate with the frequency selector system 116 and determine a difference between the target output frequency of the exemplary laser system 100 and an updated target output frequency of the exemplary laser system 100. In these variations, the control system is also operable to send a control signal to the frequency selector system 116 to generate a second selected frequency signal. The control signal is based on the difference determined by the control system, and the second selected frequency signal is generated based on the control signal. In many variations, the second selected frequency signal may have a target separated frequency comb signal that is different from that of the (first) selected frequency signal. For example, the target separated frequency comb signal of the second selected frequency signal may be closer to the updated target output frequency than the target separated frequency comb signal of the (first) selected frequency signal.

[0016] In implementations where the laser 102 is a CW laser, the laser 102 may be frequency locked to a stable frequency reference, such as an atomic or molecular absorption characteristic or an ultrastable cavity. Once locked, the laser 102 may avoid the need to relock when the output frequency of the exemplary laser system 100 is changed. Additionally, in some variations, the laser 102 may have a spectral bandwidth of less than 1 MHz. In some variations, the spectral bandwidth is less than 1 kHz, or even less than 1 Hz. Once the laser 102 is locked to an absolute frequency reference, it may operate similarly to an optical clock. In such an operating mode, the laser 102 does not need to relock to that reference when the exemplary laser system 100 is switched between different outputs.

[0017] In some implementations, the exemplary laser system 100 includes one or more amplifiers 122 that amplify the laser signal 104 or the selected frequency signal 118 after being altered by the frequency shifter 120. For example, the laser 102 (e.g., as a CW laser) can seed a high-power amplifier, which can then be used to generate a frequency comb. However, the presence of the high-power amplifier can depend on the desired output power and the characteristics of the laser 102 and the frequency comb. The frequency comb is generated at the output of the laser 102 (which itself may be amplified) and includes one or more comb lines (or frequency comb signals 118) distributed over a target frequency range. A frequency comb distribution system 112, which can include one or more dispersive elements, is applied to spatially separate the different comb lines. A desired comb line, or even a subset of comb lines, having a frequency (or frequencies) closest to the target output frequency (or set of target output frequencies) can be selected using a frequency selector system 116 (e.g., by a network of optical switches or other means). Examples of optical switch networks include those based on electro-optical switches or MEMs type switches.

[0018] The frequency comb distribution system 112 and the frequency selector system 116 may, in some variations, be combined by using a network of dropout filters on a photonic integrated circuit tuned near each comb frequency of the frequency comb. In this case, the outputs of the dropout filters may be combined in whole or in part. The output of the exemplary laser system 100 may then be determined by which dropout filters are tuned to be active. Tuning of the dropout filters may be accomplished with microheaters or by using piezoelectric elements. After the desired comb line (or frequency comb signal 118) is selected, a frequency shifter 120 may be applied to quickly and precisely shift the selected comb line to a target output frequency. By doing so, the frequency shifter 120 may be applied to fine-tune the frequency of the laser light output by the exemplary laser system 100. If multiple target output frequencies are desired, each optical channel may include a frequency shifter 120. The frequency shifter 120 can be an electro-optic modulator (eg, in a Mach-Zehnder configuration) configured as an IQ modulator, which allows for modulation of a single sideband suppressed carrier (SSB-SC).

[0019] In some implementations, the process of fine tuning is accomplished in radio frequency. For example, the tuning time of the exemplary laser system 100 may be determined by relocking a radio frequency oscillator that drives a fine tuning electro-optic modulator (e.g., a phase-locked loop in the radio frequency source). Multiple sources for fine tuning may be used to ease the relock time of the phase-locked loop in the radio frequency source. Because the laser 102 does not need to be unlocked and relocked when a different frequency is selected, the hopping time between different target frequencies may be reduced, often significantly. This reduction can be achieved while maintaining a narrow spectral bandwidth. The spectral bandwidth may be limited in some cases by the dwell time and the time before measurement at any frequency. For example, the spectral bandwidth may be limited by the time-bandwidth product, which is defined by Δν·Δt, where Δν is the bandwidth in the frequency domain and Δt is the bandwidth in the time domain (e.g., the pulse width). According to Fourier theory, the magnitude of this product can be 0.44 for a transform-limited Gaussian pulse. However, other magnitudes are possible depending on the type of pulse.

[0020] The output of the exemplary laser system 100 can be amplified to a desired output power by an optical amplifier. The optical amplifier 122 can be applied at any stage in the exemplary laser system 100, including intermediate stages for amplifying the optical signal to a level suitable for the frequency comb dispersion system 112, the frequency selector system 116, and the frequency shifter 120. Furthermore, each optical component of the exemplary laser system 100 can be optically coupled via an optical fiber or another type of optical waveguide. The exemplary laser system 100 can be incorporated into, for example, a Rydberg atom-based communication or radar system that requires fast switching times (e.g., less than 1 millisecond) between different Rydberg atom transitions. The exemplary laser system 100 can also be incorporated into a metrology system so that the detection frequency of a Rydberg atom-based electric field sensing device can be quickly changed.

[0021] In a general aspect, the laser systems described herein can be part of a Rydberg atom-based detection system. Such systems can be useful in applications where it is desirable to rapidly change the detection carrier frequency (e.g., the target RF frequency). Examples of these applications include electronic warfare, test and measurement, and telecommunications.

[0022] In some implementations, the laser system includes an optical clock, a frequency comb, a dispersive element, an optical switch, and a fine-tuning element. The frequency comb may be similar to the frequency comb generator system described in connection with FIG. 1. Similarly, the dispersive element may be similar to a frequency comb dispersive system, the optical switch may be similar to a frequency selector system, and the fine-tuning element may be similar to a frequency shifter. In these implementations, the switching speed of the laser system may be determined by the switch used and the frequency relock time of the radio frequency oscillator driving the fine-tuning element. Furthermore, the spectral linewidth may be determined by the spectral bandwidth of the optical clock, which may be less than 1 Hz in certain cases. Many atomic physics applications require laser light with a spectral bandwidth of less than 1 MHz. In some cases, such as when the laser system switches, its spectral bandwidth may be limited by the time-bandwidth product. For example, if the switching time is 1 microsecond, the laser system pauses for 10 microseconds to achieve a spectral bandwidth of 100 kHz. However, such a dwell time may be faster than the unlocking and relocking of a conventional laser. In some cases, amplifiers may be applied within the laser system or at its output to achieve the desired output power required to inject into each element throughout the system.

[0023] In some cases, the laser system includes a narrow-linewidth tunable laser. Narrow-linewidth tunable lasers can be built using external cavity diode laser (ECDL) technology, which allows for inexpensive, compact diode lasers that can generate emission in the violet to mid-infrared spectral range. With optical feedback from an external cavity, narrow-linewidth tunable lasers can also be achieved using Fabry-Perot lasers and / or gain chips. In this case, the ECDL can include a laser and a wavelength-selective component (such as a grating or interference filter), as illustrated in FIG. 2. The wavelength of a Fabry-Perot diode laser can be coarsely tuned over its wide gain bandwidth by rotating the angle between the laser beam and the grating / interference filter. Mode hops can occur when the tuning range exceeds the free spectral range of the external cavity, which in certain cases can be on the order of several GHz. However, by carefully designing the structure of the external cavity or by synchronizing the amount of injection current with the angle tuning, the mode-hop-free range can exceed 100 GHz. Temperature can be used to slowly and coarsely change the frequency, but large frequency changes may require mechanical movement of elements of the laser system (e.g. optical elements such as mirrors and lenses).

[0024] Variations in temperature and injection current of the ECDL can cause wavelength drift, while acoustic noise and other undesired perturbations of the laser cavity can cause the spectral bandwidth to exceed 1 MHz, possibly even up to 10 MHz. To compensate for these effects, active feedback loops can be applied to lock the frequency of the diode laser to a stable frequency reference, such as an atomic or molecular transition, a Fabry-Perot cavity, or an optical frequency comb. The bandwidth of the feedback loop can exceed 5 MHz, and for semiconductor lasers (e.g., ECDLs) can be around 1 MHz due to their inherent noise characteristics.

[0025] However, the laser system may include other types of lasers, such as fiber lasers, distributed feedback (DFB) lasers, or Bragg reflector (DBR) lasers. For example, fiber lasers can be tuned thermally or piezoelectrically, but need to be stabilized to a reference to eliminate laser frequency fluctuations. DFB and DBR lasers can be tuned electrically or thermally, but may also need to be stabilized to a reference for many applications. However, narrow spectral bandwidth lasers usually tune slowly (at best, over a few milliseconds) over a nm wavelength range. Furthermore, they may in some cases need to reacquire a lock signal for frequency stabilization when approaching a new target frequency.

[0026] In variations where the narrow linewidth tunable laser is an ECDL laser, the frequency of the light generated by the ECDL can be changed by changing the temperature of the ECDL. In some variations, the frequency of the light is changed by changing the current applied to the gain medium (e.g., of the ECDL or the gain chip). In some variations, the frequency is changed by changing the external cavity length of the ECDL, such as via a piezoelectric transducer. Other means are possible for changing the frequency of the light generated by the ECDL.

[0027] In some variations, the laser system includes a laser with an output frequency locked to some absolute (or near-absolute) reference. In these variations, the laser can operate similarly to an optical clock. The absolute reference can be an interferometer (e.g., a Fabry-Perot cavity) or a spectral property of a molecule or atom. However, in certain cases, the spectral property of a molecule or atom serves as a better reference because it can be linked to the structure of the atom or molecule that does not change over time. Although not an optical clock, a cesium (Cs) clock is referenced to a frequency that separates the two ground hyperfine states of a Cs atom (or a collection of Cs atoms) and can be used to stabilize optical frequency combs and lasers by using a laser optical frequency comb beat signal. Many optical clocks use narrow-band atomic or ionic transitions for the reference transition. Such a reference transition allows the laser to generate a spectroscopic signal, such as atomic absorption, that can be used to frequency lock the laser. High-speed electronics can then be used to generate an error signal from the atomic signal, which is fed back to an actuator in the laser to correct its frequency.

[0028] In some implementations, the laser (or optical clock) is configured to measure the differential signal of the atomic spectral line. The derivative of the signal can be taken (e.g., by frequency modulation spectroscopy) to generate an electrical signal with a nearly linear slope near the peak of the spectral line. If the laser is off-resonance, an electrical signal is generated that can be used to correct the output frequency of the laser. For example, the electrical signal can be a voltage signal with a sign indicating the direction of the laser frequency error. The fundamental frequency of the laser can be adjusted relative to a reference by modulating the laser, or alternatively a portion of the laser's output after it has been separated from the main laser output, to generate a sideband at a desired offset frequency and locking the sideband to the reference. The adjustment can be precise because the sideband frequency is generated by a radio frequency source that can be locked to a clock such as an ovened (or thermally stabilized) quartz oscillator or even an atomic clock. Sub-Hertz performance is possible. In many implementations of laser systems, such offset adjustment is used to align an optical frequency comb to a dispersive element such as an arrayed waveguide grating (AWG).

[0029] An optical frequency comb may be a light source with an optical spectrum defined as equally spaced spectral lines in the frequency domain. Each spectral line may be called a comb tooth and may define an equal frequency interval of modes in the optical spectrum. In this case, the optical frequency comb may be used as an optical ruler to measure unknown frequencies with a beat tone between the unknown frequency and the known comb frequency. Such measurements may allow the transfer of phase and frequency information from a high stability reference to thousands of comb teeth. Optical frequency combs can be used in applications such as frequency metrology, precision spectroscopy, and distance measurement. Furthermore, optical frequency combs may allow very agile repetition rates. For example, tuning of mode spacing in electro-optic frequency combs can benefit from high frequency, wide bandwidth, and high efficiency lithium niobate waveguide modulators.

[0030] A frequency comb can be generated (e.g., by a frequency comb generator system) using several different methods. For example, a frequency comb can be generated using a single-frequency continuous-wave laser strongly driven by an electro-optic modulator that provides tens of sidebands as comb teeth. Alternatively, a frequency comb can be generated using a mode-locked laser, which allows precise measurement of oscillations from an optical atomic clock (e.g., for precise measurement of its frequency). In this case, the optical output of the mode-locked laser can be a periodic train of optical pulses with period T, which can be expressed as a Fourier series of equally spaced optical frequencies in the frequency domain, f rep FIG. 3 illustrates an exemplary frequency comb generated from a mode-locked laser. As another example, the frequency comb can be generated using a microresonator waveguide (e.g., a so-called Kerr-type frequency comb generator). A single-frequency pump source can resonate with the microresonator waveguide. Furthermore, the single-frequency pump source can generate an output that can be converted to a frequency comb via degenerate and non-degenerate four-wave mixing processes. As another example, the frequency comb can be generated from a phase-modulated single-frequency laser.

[0031] The dispersive elements used in the laser system can be constructed in several different ways and can be part or all of a frequency comb dispersion system. For example, one way to construct a dispersive element is to use an arrayed waveguide grating (AWG), which can also be used as a demultiplexer and / or multiplexer for optical signal processing. The AWG can be constructed as a planar lightwave circuit that uses interference to separate different wavelengths of light carried in a single optical fiber into an array of optical fibers, each carrying light of a wavelength defined by the spectral bandwidth and frequency spacing of the AWG output channel.

[0032] Another optical device that provides dispersion functionality is a photonic integrated circuit incorporating a ring resonator. In a photonic integrated circuit, each ring resonator has a resonant spectral bandwidth that can be defined by engineering the Q or finesse of the ring resonator. The ring resonator can also be tuned to resonate at a specific wavelength by changing the temperature, straining the structure with a piezoelectric, or both. When integrated into a laser system, the output from the frequency comb generator system can be coupled to a photonic integrated circuit with one or more different ring resonators coupled to the output channels of the frequency comb generator system. A specific wavelength of light can be selected by tuning a specific ring resonator to resonate at the target wavelength. The light captured in the ring resonator can then be coupled to the output channels of the photonic integrated circuit.

[0033] In some variations, the photonic integrated circuit can operate as both a distributing element and a switch network. Switching the ring resonator on and off resonance at the target wavelength can be slower than can be achieved with an electro-optical switch. However, this configuration may be suitable for applications where switching speeds of tens of microseconds are acceptable. Furthermore, photonic integrated circuits that can directly select the frequency comb signal can offer certain advantages, such as improved stability and manufacturability. For example, photonic integrated circuits can enable integrated packaging that reduces the number of connections between elements of the photonic integrated circuit, and the number of fiber optic cables required for optical connections (e.g., external to the photonic integrated circuit) can also be reduced. Photonic crystal circuits can also be suitable for mass production using processes adapted from large-scale semiconductor manufacturing.

[0034] In some implementations, the switch network is based on an optical switch (or multiple optical switches). The optical switch can be used to recombine the frequency comb signals, sometimes called comb tines, and in some cases can select a particular signal or tine. For example, the optical switch can be used to select a single comb tine. Such selection can be done using dropout filters based on MEMs, electro-optical, or photonic integrated circuits. These filters can be thermally or piezoelectrically tuned. Such selection can also be done using fiber Bragg gratings when thermally or piezoelectrically tuned. Electro-optical switches can provide high-speed (e.g., less than 10 nanoseconds) switching. Optical switches using dropout filters based on photonic integrated circuits can be integrated across one or more chips (e.g., silicon dies). For example, the dropout filters can be integrated with a frequency comb generator system on one or more chips using a Kerr-type frequency comb generated by a ring resonator. In this case, the optical switch and the dispersive element can also be integrated, thereby integrating the switch network with the dispersive element. Other types of optical switches, or optical switch trees, can also be used. For some applications, low-cost, low-speed optical switches, or even shutters, can be appropriate.

[0035] To fine-tune the selected comb tines (or selected frequency signal) to a precise target frequency, the frequency shifter may include a Mach-Zehnder modulator. The spectral bandwidth of these modulators can reach as much as 40 GHz, so the comb tines can often be fine-tuned by this amount. The fine-tuning process may be done by changing the modulator frequency, which may be a radio frequency. Fine-tuning addresses the problem of relocking the optical frequency and may translate this problem into the generation of a fine-tuned radio frequency. If the switching time is shorter than the time required to relock the radio frequency, e.g., the oscillator phase-locked loop, multiple different oscillators, such as rotating (e.g., pre-programmed) frequencies for fine tuning, may be used to fine-tune the output of the laser system. The number of oscillators required may be determined, for example, by the switching time, the oscillator phase-locked loop, and the dwell time required at each frequency. The Mach-Zehnder modulator may operate as an IQ modulator in certain cases, and single-sideband suppressed carrier (SSB-SC) modulation may be used to shift the frequency of the output. SSB-SC modulation can eliminate unwanted frequencies from the frequencies output by the laser system. Improved suppression of unwanted frequencies can be achieved by using multiple Mach-Zehnder modulators (e.g., two Mach-Zehnder modulators in parallel).

[0036] The laser system may include an optical amplifier configured to amplify an optical signal from a component of the laser system. The optical signal may be generated, for example, by a laser or optical clock, a frequency comb, an optical switch, or a fine-tuning element (e.g., a modulator). In variations in which the component corresponds to a frequency comb, the optical amplifier may have a wide enough spectral bandwidth to amplify the optical signal at each comb tooth of the frequency comb. The amplification may be the same for each comb tooth. In some variations, the optical amplifier may be seeded by the power of the output from the fine-tuning element. If the power of the output from the fine-tuning element is too small, amplification may be performed in an intermediate stage. For example, the output from the frequency comb or the optical switch may be amplified to compensate for losses or low comb tooth power. It is beneficial to have sufficient seed power for the final amplification, since if the power from the weakest fine-shifted comb tooth exceeds the saturation point of the amplifier, the output power may be more uniform across the entire system spectral bandwidth. In some variations, the component corresponds to a CW laser. In these variations, the optical amplifier may amplify only at the center frequency of the CW laser. In many cases, the optical amplifier is strong enough to increase the power per comb tooth since the amplified power can be distributed among multiple comb teeth. Examples of optical amplifiers include fiber amplifiers, semiconductor optical amplifiers, and tapered amplifiers.

[0037] During operation, the wavelength of the laser system can be tuned by adjusting the angle between the laser beam and the grating (or an interference filter as illustrated in FIG. 2). Stability of the tuned wavelength can be achieved by active feedback from a frequency reference to an actuator in the laser (e.g., current for thermal heating / cooling, piezoelectric element to move the grating, etc.). To hop between different target wavelengths, the laser system may undergo a process of unlocking, wavelength tuning, and relocking. However, the time required for wavelength tuning and relocking can range from tens of milliseconds to tens of seconds, which can result in performance issues for photonic sensors, especially Rydberg atom-based sensors, for optical communications, electronic warfare, and radar applications.

[0038] In some implementations, the laser system is configured for use with a Rydberg atom-based sensor, which can increase the tunability and stability of the laser system. In this configuration, the laser system may eliminate wavelength tuning of the output of the laser. It will be appreciated that angle tuning of the grating or interference filter can be slow, especially if such tuning depends on the kHz bandwidth of the piezoelectric element. Omitting the wavelength tuning may eliminate the process of relocking the laser. In this case, the laser only needs to be locked to a single stable reference, which increases the robustness and repeatability of the system. In these configurations, the laser system may also extend the range of wavelength selection. In the case of an ECDL, the tuning achievable for the wavelength may be limited by the mode-hop-free tuning range of the ECDL. Reaching wavelengths beyond the mode-hop-free region is possible, but the temperature and injection current of the ECDL must be adjusted, which can significantly increase the time and complexity required to achieve new output wavelengths.

[0039] In many implementations, the laser system includes a narrow bandwidth laser locked to an absolute frequency reference. In such implementations, the output of this ultra-stable laser is amplified and can be used to generate a frequency comb spanning a desired wavelength range, such as by a frequency comb generator system. A dispersive element separates and routes the comb tines through a series of optical switches. If multiple precise frequencies are desired, each output of the switch tree has a fine-tuning element, such as an electro-optic modulator (EOM), to shift the output frequency to the desired position. After the switch network, the frequencies are recombined into a combined output and then amplified to the desired optical output power. If a single output frequency is desired, a single fine-tuning element is placed after the switch network. In some cases, to ensure continuous coverage, the comb spacing is smaller than the spectral range of the fine-tuning element. For example, the comb frequencies of the comb tines may be separated from each other by a common frequency spacing. Thus, the fine-tuning element may be configured to change selected comb tines by an amount of frequency equal to or greater than the common frequency spacing.

[0040] During operation, the laser system outputs highly tunable, agile, and stable laser light. The laser system can be configured to hop between different target wavelengths over tens of nanometers in less than 10 microseconds while maintaining a spectral bandwidth of less than 1 MHz. In some variations, there is no need to relock the laser system when changing wavelengths, which can increase locking robustness and reduce switching times.

[0041] FIG. 4 depicts an example of a laser system, which in this particular example includes four components. The laser system includes a CW laser with a narrow linewidth. The CW laser is actively locked to a frequency reference. Furthermore, the frequency (or wavelength) of the CW laser is fixed to a single value that does not change appreciably in many cases. Examples of CW lasers include semiconductor lasers, solid-state lasers, and fiber lasers. The frequency reference is configured to provide an optical signal whose frequency is defined by a precise value. Modulating the CW laser around the reference frequency generates a feedback signal that allows the frequency of the CW laser to be stabilized to the frequency reference. Examples of frequency references include an optical wavemeter, an ensemble of atoms or molecules, or a stable Fabry-Perot cavity. If the output power of the CW laser is too low to generate enough optical power in the comb line, or if additional optical power is required to lock the CW laser to the frequency reference, an optical amplifier may be used.

[0042] An exemplary laser system may also include a frequency comb. The output of a CW laser (or an amplifier, if present) may be used to generate a frequency comb across the target wavelength spectrum, with a comb tooth spacing of tens of GHz. The comb spacing may be smaller or larger than this figure. However, in many applications, it is beneficial to make the comb tooth spacing as large as possible (considering the availability of RF devices) to span the largest spectral range. In some cases, the tooth spacing may have an upper limit set by the ability of the fine-tuning elements to continuously cover the range between the comb teeth. The fine-tuning elements may, for example, fine-tune the output by one full comb spacing (or ± half a comb spacing). In some variations, the frequency comb is generated by using a four-wave mixing process in a photonic crystal fiber, four-wave mixing in a microresonator (Kerr frequency comb), or electro-optic modulation.

[0043] The exemplary laser system may further include a frequency filter. The frequency filter is configured to generate selected comb teeth from the frequency comb. In certain cases, the output of the frequency comb is filtered, thereby outputting only one comb tooth from the frequency filter that is closest to the target frequency. If multiple output frequencies are desired, the frequency filter is used to select a subset of the comb teeth to be recombined at the output of the system. A thermally or piezoelectrically tuned fiber Bragg grating or ring resonator may be used as an optical bandpass filter to select the target comb teeth. Alternatively, the frequency filter may include an arrayed waveguide grating (AWG) optically coupled to an optical switch. The comb teeth of the frequency comb are diffracted into individual optical channels through the arrayed waveguide grating, and the optical switch selects the optical channel or channels that are closest to the target frequency.

[0044] An exemplary laser system may also include fine tuning elements (e.g., high-speed frequency shifters). For example, a high-speed electro-optic modulator may be applied to fine-tune and rapidly shift the selected comb tines to the target frequency. Such shifting may also include scanning the selected comb tines within a spectral bandwidth around the target frequency. Additionally, single-sideband suppressed carrier (SSB-SC) modulation may be used so that a single frequency is output. In this case, the electro-optic modulator may be a Mach-Zehnder type modulator. A high-bandwidth lithium niobate waveguide modulator may be used for a wide range of frequencies. In some cases, an optical amplifier may be used to boost the power of the final output.

[0045] During operation, the CW laser can remain locked to a frequency reference. Such locking can eliminate the process of wavelength tuning in the CW laser and may also eliminate the process of relocking. Frequency combs can be generated over tens to hundreds of nanometers, which is well beyond the mode-hop-free tuning range of an ECDL. Alternatively, the CW laser can be stabilized by locking one of the comb teeth to a frequency reference rather than the direct output of the CW laser. This type of locking expands the allowable wavelength range when no frequency reference exists close to the center of the desired wavelength range.

[0046] 5, a schematic diagram of an exemplary laser system including a frequency reference defined by a water vapor cell is depicted. In some implementations, the exemplary laser system is configured to generate light at approximately 1455 nm and is tunable over a bandwidth of 1560 GHz. The switching time of the exemplary laser system is less than 10 microseconds, and the exemplary laser system can be programmed to hop between different precise (e.g., 1 MHz or less) frequencies.

[0047] An exemplary laser system includes a CW laser composed of a tunable laser gain chip and an external cavity diode laser (ECDL). For example, FIG. 6 represents a photograph of an exemplary ECDL including a piezoelectric mounted mirror, a tunable laser gain chip, and angled optics that act as an interference filter. FIG. 7 represents a schematic diagram of the exemplary ECDL of FIG. 6 illustrating the optical path extending from the piezoelectric mounted mirror through the tunable laser gain chip and angled optics. The wavelength of the ECDL is determined by the angle between the laser beam and the interference filter. The CW laser has a mirror mounted on a piezoelectric mount and controls the cavity length for slow feedback (e.g., less than 5 kHz) and the gain chip current for fast feedback (e.g., less than 5 kHz). The output power of the CW laser is about 30 mW. A semiconductor optical amplifier can be used to increase the output power of the ECDL and lock it to a frequency reference to seed a high power amplifier for the generation of a frequency comb. The output wavelength is about 1455.5 nm. The ECDL can be directly locked to a Fabry-Perot cavity or to the spectral properties of atoms or molecules. In certain cases, the ECDL may also be offset locked and tuned to any frequency around a reference, which may include a tolerance range of hundreds of MHz to GHz depending on the reference.

[0048] The exemplary laser system also includes a frequency reference whose transition lines are defined by the water vapor in the vapor cell. The exemplary laser system can also include a single frequency Raman fiber amplifier capable of generating approximately 5 W of power. The amplifier can amplify the output of the exemplary laser system to 1 W or more of power. The frequency comb is generated by cascading the intensity of a waveguide electro-optic and a phase modulator with a comb tooth spacing of approximately 20 GHz. The waveguide electro-optic is based on a lithium niobate material (e.g., a single crystal of lithium niobate). The exemplary laser system includes one intensity modulator and three phase modulators to generate at least 60 comb lines spanning a frequency spectrum of over 1200 GHz. In some variations, the intensity modulator is used to smooth the changes in the amplitude of the comb teeth relative to each other. However, in other variations, it is not necessary to include an intensity modulator.

[0049] An exemplary laser system may include other numbers of intensity and phase modulators to generate comb lines. For example, FIG. 8 represents a graph of an exemplary frequency comb generated by one intensity modulator and one phase modulator. This exemplary frequency comb includes 18 individual teeth, the presence of which is indicated by blue boxes on the graph. The tooth size count is indicated on the ordinate on the left side of the graph. Each additional phase modulator adds an additional line (e.g., as many as the applied power, which is shown here to be about 1 watt). The intensity modulator is used to flatten the intensity across the exemplary frequency comb, so that there is not much variation in intensity between comb teeth.

[0050] The exemplary laser system further includes an arrayed waveguide grating having 80 channels, an example of which is illustrated in FIG. 9. The AWG is configured to diffract each comb tooth into an independent channel, providing a span of 1600 GHz across the spectral band of interest. The output of the frequency comb is coupled to the input of the AWG via a fiber optic cable. Each of the 80 output channels of the AWG is coupled to a fiber optic cable. The center frequency of the AWG can be tuned by temperature. Additionally, the frequency comb can be aligned to the AWG transmission peak. Such alignment can be achieved by combining the frequency comb with an offset lock of the ECDL as well as utilizing an electro-optically generated frequency comb to control the comb tooth spacing. Using the AWG, the exemplary laser system can have a band spacing of 19.4 GHz and a center frequency of 1455.191 nm.

[0051] The exemplary laser system also includes an optical switch incorporating a PLZT-based electro-optic element. The optical switch may be configured, for example, to have 1×80 channels, as illustrated in FIG. 9, where PLZT has the composition [Pb 1-x La x ][Zr y Ti 1-y ] 1-x / 4 It may refer to a class of transparent oxide materials represented by O3 (e.g., x=0.08 and y=0.65). The optical switch has a response time on the order of nanoseconds (e.g., less than 100 nanoseconds) and can be switched between the output channels of the AWG, causing one channel to be emitted from the optical switch. This high-speed electro-optical switch can be configured as a tree of sub-switches where 80 frequency comb signals enter the switch and a selected one frequency comb signal exits the switch. For example, one output can exit the switch by activating a selected number of the sub-switches. The output is optically coupled to a fine-tuning element, such as an electro-optical modulator (EOM). The selected frequency comb signal can thus define the selected frequency signal of the fine-tuning element. In many variations, the switching time of the optical switch is less than 10 nanoseconds.

[0052] The exemplary laser system includes a Mach-Zehnder IQ waveguide electro-optic modulator based on lithium niobate material. The electro-optic modulator acts as a fine tuning element and can be used to shift the frequency of the overall output of the exemplary laser system to a target output frequency. For example, the selected comb tines (or selected frequency signal) can be close to the target frequency. The electro-optic modulator can be used to shift the frequency of the selected comb tines to the target output frequency (e.g., atomic or molecular transitions). SSB-SC modulation can be used to suppress or eliminate undesired frequencies. Frequency switching can be limited in some cases by relocking the radio frequency oscillator.

[0053] The exemplary laser system further includes one or more semiconductor and / or fiber optic amplifiers at the output of the fine-tuning element or the exemplary laser system, configured to boost the output power of the exemplary laser system to at least 100 mW. FIG. 9 depicts a pair of amplifiers (e.g., a preamplifier and an amplifier) ​​arranged in series. However, other arrangements are possible. For example, both amplifiers can be arranged in parallel. In this arrangement, both amplifiers are fiber-coupled to the output of the fine-tuning element on one side and to each other on the other side to define the output of the exemplary laser system.

[0054] The exemplary laser system may further include a control system (e.g., a computer, microprocessor, or other type of system) configured to control the operation of the FPGA. The FPGA may be part of the control system and may include a program for locking the ECDL. The FPGA may also include a program for controlling components of the exemplary laser system, such as an optical switch. For example, the optical switch may be preprogrammed to generate a series of output frequencies. The fine-tuning element may also be preprogrammed such that an optical signal therefrom has a frequency corresponding to each of the series of output frequencies. The fine-tuning element may output a series of frequencies with appropriate dwell times for each particular application. The control system may also be programmed to monitor environmental conditions of the exemplary laser system and accordingly adapt the laser lock and fine-tuning parameters to the environmental conditions. In some implementations, a stabilized clock, a GPS, or a GPS-operated clock may be used to stabilize the fine-tuning frequency as well as the interval in the frequency comb. Such latter fine-tuning frequency may be an RF frequency generated by an oscillator built into the exemplary laser system and referenced to an ovened (or thermally stabilized) crystal oscillator.

[0055] In some implementations, the laser system may include a photonic integrated circuit element. For example, FIG. 10 depicts a schematic diagram of an exemplary laser system including a photonic integrated circuit element for generating a frequency comb and filtering an optical signal. This exemplary laser system includes an ECDL locked to a Fabry-Perot cavity that may define an ultra-stable optical cavity. Light is amplified at the output of the ECDL (which is part of a CW laser) and then frequency doubled, such as by a nonlinear optical crystal. Such doubling may be beneficial when the manipulation of optical signals by frequency combs and optical switches is easier at infrared frequencies, but a visible light output is desired. In some cases, the ECDL generates a 1018 nm laser light that is amplified and frequency doubled to a 509 nm optical signal. This optical signal may provide a combined laser wavelength that can be used for a vapor cell sensor (e.g., a two-photon Rydberg atomic sensor).

[0056] In some aspects of the described subject matter, the laser system may be described by the following examples. Example 1. A laser system comprising: a laser configured to generate a laser signal; a frequency comb generator system configured to generate a frequency comb based on a laser signal, the frequency comb comprising a frequency comb signal at a respective comb frequency; a frequency comb distribution system configured to spatially separate the frequency comb signal into respective optical channels of the frequency comb distribution system; a frequency selector system configured to generate a selected frequency signal from the frequency comb signal after separation, the selected frequency signal comprising the target separated frequency comb signal; a frequency shifter configured to change the selected frequency signal toward a target output frequency of the laser system; Equipped with. Example 2. The laser system of example 1, wherein the target isolated frequency comb signal has a comb frequency closest to the target output frequency. Example 3. The laser system of example 1, wherein the target isolated frequency comb signal has a comb frequency that is second closest to the target output frequency. Example 4. The laser system of example 1 or any one of examples 2-3, wherein the frequency shifter is configured to modify the selected frequency signal by fine-tuning the selected frequency signal to a target output frequency. Example 5. The laser system of example 1 or any one of examples 2-4, wherein the frequency shifter is configured to vary the selected frequency signal by scanning the selected frequency signal around a target output frequency. Example 6. The laser system of any one of Examples 1 or 2 to 5, the laser is a continuous wave laser; The laser signal is the frequency locked output of a continuous wave laser. Example 7. The laser system of example 6, a vapor cell having vapor therein; The frequency locked output is based on electronic transitions of the vapor. Example 8. The laser system of example 6, an optical cavity having an optical path length; The frequency locking output is based on the optical path length of the optical cavity. Example 9. The laser system of any one of examples 1 or 2-8, wherein the frequency comb dispersion system comprises: an input optical channel for receiving a frequency comb signal from a frequency comb generator system; a plurality of output optical channels that are physically separated from each other by a fixed distance; The arrayed waveguide grating has Example 10. The laser system of any one of Examples 1 or 2 to 9, The frequency selector system comprises a network of optical switches; The laser system includes a control system configured to control the network of optical switches to select a target optical channel carrying a target isolated frequency comb signal. Example 11. The laser system of example 1 or any one of examples 2-8 and example 10, comprising a photonic integrated circuit comprising a network of dropout filters to define a frequency comb dispersion system. Example 12. The laser system of example 11, wherein the photonic integrated circuit further defines a frequency selector system. Example 13. The laser system of example 11 or example 12, wherein the network of dropout filters is defined by a plurality of ring resonators associated with respective frequency comb signals of the frequency combs, each ring resonator configured to resonate at a ring frequency when an optical path length of the ring resonator reaches a target optical path length, the ring frequency coinciding with the comb frequency of one of the frequency comb signals. Example 14. The laser system of example 1 or any one of examples 2-13, comprising one or more amplifiers that amplify the laser signal or amplify the selected frequency signal after it has been altered by the frequency shifter. Example 15. The laser system of example 1 or any one of examples 2-14, wherein the frequency shifter comprises an optical modulator performing single sideband suppressed carrier (SSB-SC) modulation. Example 16. The laser system of any one of Example 1 or Examples 2 to 15, in communication with a frequency selector system; determining a difference between a target output frequency of the laser system and an updated target output frequency of the laser system; transmitting a control signal based on the difference to a frequency selector system to generate a second selected frequency signal; The device further comprises a control system configured to perform operations comprising: Example 17. The laser system of any one of examples 1 or 2-16, The comb frequencies of the frequency comb signal are separated from one another by a common frequency interval, The frequency shifter is configured to change the selected frequency signal by a frequency magnitude equal to or greater than the common frequency interval.

[0057] In some aspects of the described subject matter, a method for tuning a laser system may be described by the following example. Example 18. A method for conditioning an optical signal, the method comprising: generating a frequency comb having frequency comb signals at respective comb frequencies, the frequency comb being based on a laser signal; Spatially separating the frequency comb signals into respective optical channels of a laser system; generating a selected frequency signal from the separated frequency comb signal, the selected frequency signal comprising a target separated frequency comb signal; modifying the selected frequency signal toward a target output frequency of the laser system; Includes. Example 19. The method of example 18, wherein the target isolated frequency comb signal has a comb frequency closest to the target output frequency. Example 20. The method of example 18, wherein the target isolated frequency comb signal has a comb frequency that is second closest to the target output frequency. Example 21. The method of example 18 or any one of examples 19-20, wherein modifying the selected frequency signal includes fine-tuning the selected frequency signal to a target output frequency. Example 22. The method of Example 18 or any one of Examples 19-21, wherein varying the selected frequency signal includes scanning the selected frequency signal around the target output frequency. Example 23. The method of any one of Example 18 or Examples 19 to 22, generating a laser signal by operating a laser; communicating a laser signal from the laser to a frequency comb generator system of the laser system, the frequency comb generator system operable to generate a frequency comb; Includes. Example 24. The method of example 23, amplifying a laser signal; amplifying the selected frequency signal after the selected frequency signal is changed; Includes one or both of the following: Example 25. The method of example 23 or example 24, the laser is a continuous wave laser; The laser signal is the frequency locked output of a continuous wave laser. Example 26. The method of example 25, wherein the frequency locked output is based on electronic transitions of a vapor in the vapor cell. Example 27. The method of example 25, wherein the frequency locking output is based on an optical path length of the optical cavity. Example 28. The method of any one of Example 18 or Examples 19 to 27, The laser system includes an arrayed waveguide grating having an input optical channel and a plurality of output optical channels physically spaced apart from one another; Spatially separating the frequency comb signals Receiving a frequency comb signal at an input optical channel of an arrayed waveguide grating; Separating each frequency comb signal into a separate output optical channel of an arrayed waveguide grating; communicating respective frequency comb signals from the plurality of output optical channels to a frequency selector system of the laser system; wherein the frequency selector system is operable to generate a selected frequency signal. Example 29. The method of example 18 or any one of examples 19-28, The laser system is A network of optical switches; a control system configured to control the network of optical switches; Equipped with The method includes selecting, by operation of a control system, a target optical channel carrying a target isolated frequency comb signal. Example 30. The method of example 18 or any one of examples 19-27 and 29, wherein the frequency comb signal is spatially separated into respective optical channels by a photonic integrated circuit comprising a network of dropout filters. Example 31. The method of example 30, wherein the selected frequency signal is generated by a photonic integrated circuit. Example 32. The method of example 30 or example 31, the network of dropout filters corresponds to a plurality of ring resonators associated with respective frequency comb signals of the frequency comb; Spatially separating the frequency comb signals includes altering an optical path length of a ring resonator to a target optical path length to cause the ring resonator to resonate at a ring frequency, where the ring frequency corresponds to the comb frequency of one of the frequency comb signals. Example 33. The method of example 18 or any one of examples 19-32, wherein modifying the selected frequency signal includes modulating the selected frequency signal in accordance with single sideband suppressed carrier (SSB-SC) modulation. Example 34. The method of any one of Example 18 or Examples 19 to 33, The laser system includes a frequency selector system operable to generate a selected frequency signal; The method is: determining, by operation of the control system, a difference between a target output frequency of the laser system and an updated target output frequency of the laser system; transmitting a control signal to a frequency selector system based on the difference by operation of a control system; Includes. Example 35. The method of example 34, Generating a second selected frequency signal based on the control signal by operation of the frequency selector system. Example 36. The method of any one of Example 18 or Examples 19 to 35, The comb frequencies of the frequency comb signal are separated from one another by a common frequency interval, Modifying the selected frequency signals includes modifying the selected frequency signals by a frequency magnitude greater than or equal to the common frequency interval.

[0058] Although the present specification contains many details, these should not be understood as limitations on the scope that may be claimed, but as descriptions of features specific to particular examples. Certain features described herein or illustrated in the drawings in the context of separate implementations may also be combined. Conversely, various features that are described or illustrated in the context of a single implementation may also be implemented in multiple embodiments separately or in any suitable subcombination.

[0059] Similarly, although operations are depicted in the figures in a particular order, this should not be understood as requiring such operations to be performed in the particular order or sequence shown, or as requiring all of the illustrated operations to be performed, in order to achieve desired results. In certain circumstances, multitasking and parallel processing may be advantageous. Furthermore, the separation of various system components in the above implementations should not be understood as requiring such separation in all implementations, and it should be understood that the program components and systems described may be integrated into a single product or packaged into multiple products.

[0060] A number of embodiments have been described. However, it will be understood that various modifications may be made. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. 1. A laser system comprising: a laser configured to generate a laser signal; a frequency comb generator system configured to generate a frequency comb based on the laser signal, the frequency comb comprising a frequency comb signal at a respective comb frequency; and a frequency comb distribution system configured to spatially separate the frequency comb signal into respective optical channels of the frequency comb distribution system; a frequency selector system configured to generate a selected frequency signal from the frequency comb signal after separation, the selected frequency signal comprising a target separated frequency comb signal; a frequency shifter configured to change the selected frequency signal toward a target output frequency of the laser system; A laser system comprising:

2. 10. The laser system of claim 1, wherein the target isolated frequency comb signal has a comb frequency closest to the target output frequency.

3. 10. The laser system of claim 1, wherein the frequency shifter is configured to modify the selected frequency signal by fine-tuning the selected frequency signal to the target output frequency.

4. 10. The laser system of claim 1, wherein the frequency shifter is configured to vary the selected frequency signal by scanning the selected frequency signal around the target output frequency.

5. the laser is a continuous wave laser; the laser signal is a frequency locked output of the continuous wave laser; 10. The laser system of claim 1 .

6. a vapor cell having vapor therein; The frequency lock output is based on an electronic transition of the vapor.

6. The laser system of claim 5.

7. an optical cavity having an optical path length; the frequency locking output is based on the optical path length of the optical cavity; 6. The laser system of claim 5.

8. The frequency comb dispersion system includes: an input optical channel for receiving the frequency comb signal from the frequency comb generator system; a plurality of output optical channels that are physically separated from each other by a fixed distance; 8. The laser system according to claim 1, comprising an arrayed waveguide grating having:

9. the frequency selector system comprises a network of optical switches; the laser system comprising a control system configured to control the network of optical switches to select a target optical channel carrying the target isolated frequency comb signal. The laser system according to any one of claims 1 to 7.

10. A laser system according to any one of claims 1 to 7, comprising a photonic integrated circuit comprising a network of dropout filters and defining said frequency comb dispersion system.

11. The laser system of claim 10 , wherein the photonic integrated circuit further defines a frequency selector system.

12. 11. The laser system of claim 10, wherein the network of dropout filters is defined by a plurality of ring resonators associated with respective frequency comb signals of the frequency combs, each ring resonator configured to resonate at a ring frequency when an optical path length of the ring resonator reaches a target optical path length, the ring frequency coinciding with the comb frequency of one of the frequency comb signals.

13. The laser system of any one of claims 1 to 7, wherein the frequency shifter comprises an optical modulator performing Single Sideband Suppressed Carrier (SSB-SC) modulation.

14. in communication with the frequency selector system; determining a difference between the target output frequency of the laser system and an updated target output frequency of the laser system; transmitting a control signal based on the difference to the frequency selector system to generate a second selected frequency signal; 8. The laser system of claim 1, further comprising a control system configured to perform operations comprising:

15. the comb frequencies of the frequency comb signal are separated from one another by a common frequency interval; the frequency shifter is configured to modify the selected frequency signal by a frequency magnitude equal to or greater than the common frequency interval; The laser system according to any one of claims 1 to 7.

16. 1. A method for tuning a laser system, comprising: generating a frequency comb having frequency comb signals at respective comb frequencies, the frequency comb being based on a laser signal; spatially separating the frequency comb signals into respective optical channels of the laser system; generating a selected frequency signal from the separated frequency comb signal, the selected frequency signal comprising a target separated frequency comb signal; modifying the selected frequency signal toward a target output frequency of the laser system; A method comprising:

17. The method of claim 16 , wherein the target isolated frequency comb signal has a comb frequency closest to the target output frequency.

18. 18. The method of claim 16 or 17, wherein modifying the selected frequency signal comprises fine-tuning the selected frequency signal to the target output frequency.

19. 18. The method of claim 16 or 17, wherein varying the selected frequency signal comprises scanning the selected frequency signal around the target output frequency.

20. generating said laser signal by operating a laser; communicating the laser signal from the laser to a frequency comb generator system of the laser system, the frequency comb generator system operable to generate the frequency comb; 18. The method of claim 16 or 17, comprising:

21. the laser is a continuous wave laser; the laser signal is a frequency locked output of the continuous wave laser; the frequency-locked output is based on electronic transitions of vapor in a vapor cell; 21. The method of claim 20.

22. the laser is a continuous wave laser; the laser signal is a frequency locked output of the continuous wave laser; the frequency locking output is based on the optical path length of the optical cavity; 21. The method of claim 20.

23. the laser system comprises an arrayed waveguide grating having an input optical channel and a plurality of output optical channels physically spaced a fixed distance from one another; Spatially separating the frequency comb signals includes: receiving the frequency comb signal at the input optical channel of the arrayed waveguide grating; splitting each frequency comb signal into a separate output optical channel of the arrayed waveguide grating; communicating respective frequency comb signals from the plurality of output optical channels to a frequency selector system of the laser system; wherein the frequency selector system is operable to generate the selected frequency signal.

18. The method of claim 16 or 17.

24. The laser system includes: A network of optical switches; a control system configured to control the network of optical switches; Equipped with the method includes selecting, by operation of the control system, a target optical channel carrying the target isolated frequency comb signal; 18. The method of claim 16 or 17.

25. 18. The method of claim 16 or 17, wherein the frequency comb signal is spatially separated into respective optical channels by a photonic integrated circuit comprising a network of dropout filters.

26. 26. The method of claim 25, wherein the selected frequency signal is generated by the photonic integrated circuit.

27. the network of dropout filters corresponds to a plurality of ring resonators associated with respective frequency comb signals of the frequency comb; and spatially separating the frequency comb signals includes altering an optical path length of a ring resonator to a target optical path length to cause the ring resonator to resonate at a ring frequency, the ring frequency coinciding with the comb frequency of one of the frequency comb signals.

26. The method of claim 25.

28. 18. The method of claim 16 or 17, wherein modifying the selected frequency signal comprises modulating the selected frequency signal in accordance with Single Sideband Suppressed Carrier (SSB-SC) modulation.

29. the laser system comprising a frequency selector system operable to generate the selected frequency signal; The method comprises: determining, by operation of a control system, a difference between the target output frequency of the laser system and an updated target output frequency of the laser system; transmitting a control signal to the frequency selector system based on the difference through operation of the control system; generating a second selected frequency signal based on the control signal by operation of the frequency selector system; 18. The method of claim 16 or 17, comprising:

30. the comb frequencies of the frequency comb signal are separated from one another by a common frequency interval; modifying the selected frequency signals includes modifying the selected frequency signals by a frequency magnitude equal to or greater than the common frequency interval.

18. The method of claim 16 or 17.

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