Laser systems for spectroscopy

Abstract

The implementations described here are spectroscopy systems that provide frequency-, amplitude-, and power-stabilized light to a vapor cell. An optical signal can be split into two optical paths: the first path includes an acousto-optic modulator (AOM) to modulate the frequency and amplitude and generate a pump optical signal, and the second path includes a variable optical attenuator (VOA) to generate a probe optical signal. These optical signals can then be fed into a vapor cell (also called a gas cell) to perform spectroscopy.

Description

Laser systems for spectroscopy Background Field The embodiments of the present invention relate generally to a laser system for performing spectroscopy. Description of the related art Optical atomic clocks offer improved frequency instabilities compared to microwave frequency standards due to the superior Q factor associated with optical resonance. To take advantage of these high-quality factors, coherent light-matter interaction is required. A barrier to the widespread deployment of optical frequency standards is the need to develop compact, robust, and low-power laser sources that can be integrated at the optical frequency of interest. Additionally, the laser system must provide a means of controlling the systematic errors typically associated with optical frequency standards, including residual amplitude modulation (RAM) and AC light shifts. To eliminate the first-order Doppler effects associated with interrogating hot atomic vapor, laser systems typically employ modulation transfer spectroscopy (MTS) or frequency modulation spectroscopy (FMS). However, these approaches often rely on multiple expensive and power-intensive optical components, such as acousto-optic modulators (AOMs) and electro-optic modulators (EOMs). For example, previously demonstrated MTS techniques begin with two distinct optical beams—pump and probe signals—where each beam passes through its own AOM. The pump AOM beam creates both a static frequency shift from the probe and imparts a modulation. Often, the laser system requires frequency doubling to match the sample wavelength, adding complexity to generating pump and probe beams with appropriate characteristics.Each beam can pass through a respective second harmonic generator (SHG) module to change the wavelength of the signals. MTS spectroscopy is performed using the two resulting beams. However, providing AOM and SHG modules in both optical paths adds substantial cost, volume, and power consumption to the system. Generating two spectroscopy beams with characteristics suitable for MTS is challenging in a simple laser system due to the requirement that the probe beam be free of spurious modulation from the pump light or control electronics. Furthermore, reliable RAM reduction in the pump beam is complicated by the polarization properties of the modulators. Fiber delivery of the beams to the atomic configuration is ideal in many cases; however, fiber exacerbates the potential for unwanted modulated light in the probe beam path through the etalons, and inadequate RAM control through polarization variation over temperature and time in the fibers. The document "OPTICAL FREQUENCY STANDARD AT 532 NM" by Eickhoff ML et al. discloses a double-frequency Nd:YAG laser source. The paper "Semiconductor laser active frequency stabilization technologies: a review" by Wu Yue et al. discloses the basic principles, characteristics, and latest research advances on various types of semiconductor lasers and active frequency stabilization techniques based on different locking reference standards. The paper "Doppler-free two-photon modulation transfer spectroscopy in sodium dimers" by Ma LS et al. reports on modulation transfer spectroscopy in the two-photon transition of Na2 using a CW dye laser. The paper "Improve the frequency stabilization of Cs fountain clock's optical system with modulation transfer spectroscopy" by Bai Yang et al. discloses how to arrange two sets of modulation transfer spectroscopy apparatus to stabilize two independent lasers. The document US 4590 597 A discloses how to accurately stabilize a laser at a sub-Doppler resonance. Summary The problems mentioned above are overcome by the characteristics of independent claims. Brief description of the drawings To better understand the aforementioned characteristics of this disclosure, a more detailed description of the disclosure, briefly summarized above, may be provided by reference to the embodiments, some of which are illustrated in the accompanying drawings. However, it should be noted that the accompanying drawings illustrate only illustrative embodiments and, therefore, should not be considered limiting of its scope; other equally effective embodiments may be included. Figures 1A-1C illustrate spectroscopy systems, according to one embodiment. Figure 2 illustrates a spectroscopy system, according to one embodiment. Figure 3 is a flowchart for preparing optical signals for spectroscopy, according to one embodiment. Figures 4A and 4B illustrate laser systems for performing spectroscopy, according to one embodiment. Figure 5 illustrates a spectroscopy system, according to one embodiment. To facilitate understanding, identical reference numbers have been used, where possible, to designate identical elements common to the figures. It is envisaged that the elements and features of one embodiment may be beneficially incorporated into other embodiments without further recitation. Detailed description The embodiments described herein are laser systems that provide frequency, amplitude, and power-stabilized light using a single acoustic-optical modulator (AOM) or omit the AOM entirely. The laser system is adaptable to MTS and FMS spectroscopy schemes. The laser system may include an optical source with a frequency doubler (SHG module) that emits an unmodulated optical signal. This signal can then be split into two optical paths: the first path includes an AOM that performs frequency and amplitude modulation to generate a pumping optical signal, and the second path generates a probe optical signal. These optical signals can then be transmitted to a vapor cell (also called a gas cell) for spectroscopy.Advantageously, such a laser system can generate the pump and probe optical signals using only one SHG module and one AOM, thereby reducing costs and space in the laser system compared to previous solutions. The laser system reduces the potential for contamination of the probe's optical signal with modulated light and enables reliable RAM control of the pump's optical signal, which is insensitive to the modulator's input polarization. These features allow for robust configurations of fiber-based laser systems. In particular, for implementations where the optical source requires wavelength adjustment, such as frequency doubling, the laser systems described below can use only a single SHG module, thereby reducing cost and size. Furthermore, the optical source (e.g., an optical amplifier coupled to a laser) can be controlled to make DC adjustments to the overall optical power, in contrast to other solutions that rely on the AOM to control the total power of the optical signals. Transferring this responsibility to the optical source means that the AOM only needs to make AC changes to the pump signal, which is beneficial for long-term operation. A variable optical attenuator (VOA) can make additional DC power adjustments at the probe or pump optical signals to ensure a proper power ratio between the two optical paths. In one embodiment, the spectroscopy system includes a control system that performs digital synthesis to generate a control signal for the AOM.The control system (e.g., an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA)) can output a control signal that includes a frequency shift between the pump and probe optical signals, a frequency modulation component to diffuse the pump optical signal, and an amplitude modulation component to compensate for unwanted amplitude modulation in the pump optical signal. The use of digital synthesis (as opposed to analog circuitry) improves signal quality, eliminates unwanted intermodulation mixing products, reduces potential crosstalk from modulating multiple devices on the same baseband frequency, and eliminates any analog mixing circuitry. Figure 1A illustrates a spectroscopy system 100, according to one embodiment. The system 100 includes an optical source 105 that generates an unmodulated optical signal 110. For example, the optical source 105 may be a single laser or a laser system. For example, the optical source 105 may represent a system of components, which is described in Figure 4A below. In one embodiment, the optical source 105 includes a frequency converter such as an SHG module (also called a frequency doubler) that adjusts the wavelength of the optical signal generated by the source 105. For example, a laser in the optical source 105 may generate an optical signal with a wavelength of 1064 nm, which is then frequency-doubled by an SHG module to generate an unmodulated optical signal 110 of 532 nm.However, if the optical source 105 generates an optical signal at the desired frequency of the spectroscopy system 100, then a frequency converter of the optical source 105 can be omitted. The unmodulated optical signal 110 is received at a divider 115 which divides the power in the signal 110 between two optical paths. The embodiments in this document are not limited to any particular division ratio (e.g., 50 / 50, 60 / 40, etc.). The upper optical path includes an AOM 120 which, in one embodiment, performs both frequency and amplitude modulation on the unmodulated signal 110 to generate a modulated optical pump signal 130. Frequency modulation can be performed to diffuse the optical pump signal 130 to help the system 100 lock the optical source 105 to a resonant frequency. Amplitude modulation can be performed to counteract unwanted amplitude modulation (e.g., residual amplitude modulation (RAM)) that occurs in the AOM or the optical path before the vapor cell 140. In one example, some frequency modulation is desired (to perform the oscillation), but not amplitude modulation. Therefore, the AOM 120 can perform amplitude modulation that is out of phase with the amplitude modulation that is imprinted on the optical pump signal 130 as it passes from the AOM to the vapor cell.The use of digital synthesis to perform frequency and amplitude modulation with the AOM 120 is described in more detail in Figure 5. In one embodiment, the AOM 120 does not perform DC power adjustments to the pumping optical signal 130; these are instead performed by the optical source 105. That is, the AOM 120 can only make AC adjustments to the optical signal 130 by performing both frequency and amplitude modulation. This is advantageous because AOM 120s can typically only perform small DC power adjustments to optical signals, but are very efficient at performing AC adjustments. Therefore, shifting the responsibility for DC power adjustments for the entire system 100 to the optical source 105 can reduce the load on the AOM 120. This allows the AOM 120 to perform both frequency and amplitude modulation, whereas previous solutions involved separate electro-optical modulators (EOMs) in the same path as the AOM 120 to generate a pumping optical signal. In other words, in this embodiment, there is no EOM arranged between the AOM 120 and the steam cell 140. The lower optical path includes a VOA 125 that adjusts the power of the received unmodulated optical signal 110 to generate a power-controlled probe optical signal 135. Unlike the pump optical signal 130, in this embodiment, the probe optical signal 135 is not frequency modulated. In one embodiment, the VOA 125 performs DC power control on the probe optical signal 135 to establish a desired power ratio between the optical signals in the upper and lower optical paths. This can be done to compensate for stability issues with the splitter 115 or the delivery optics after the splitter, such as optical fiber. In one embodiment, the VOA 125 includes a mirror that is adjusted to control the output power of the probe optical signal 135. While the figures illustrate the placement of the VOA 125 in the path of the probe's optical signal 135, in another embodiment the VOA 125 is arranged in the path of the pump signal 130 to control the DC power of the pump signal, while the optical source 105 is used to stabilize the probe's beam power. In that scenario, there would be no VOA between the splitter 115 and the vapor cell 140. Advantageously, power control of the probe optical signal 135 can be achieved using the VOA 125, which is less expensive, consumes less power, and is typically a smaller component than an AOM. Therefore, the upper and lower optical paths in the system 100 include only an AOM 120 and avoid having a separate EOM to perform frequency modulation on the pump optical signal 130. The optical signals from pump and probe 130, 135 are received in a vapor cell 140. The vapor cell 140 is a container (e.g., a tube) that contains atoms or molecules (e.g., iodine, acetylene, or rubidium vapor, atomic beams, laser-cooled atoms) that have a well-defined absorption spectrum. At least a portion of the sides or walls of the vapor cell 140 is transparent so that the optical signals can enter and exit the cell 140. Light absorption or fluorescence can be measured while changing the wavelength of the optical signal from pump 130 that is transmitted through the vapor cell 140, resulting in a peak or a series of peaks. At a peak, the pump optical signal 130 is absorbed by the vapor in cell 140, while at other wavelengths, the optical signal 130 passes through the vapor cell 140 with minimal absorption. In this embodiment, the steam cell 140 has two inlets that receive the optical signals from pump and probe 130, 135. In one embodiment, the optical signals 130, 135 from pump and probe pass through the steam cell 140 along optical paths that overlap at least partially. That is, at least a portion of the optical paths of the optical signal from pump 130 and the optical signal from probe 135 in the steam cell 140 are aligned (e.g., on the same axis). For example, mirrors can be used so that signals 130 and 135 can enter at different locations along the same side of the vapor cell 140 and still have optical paths that align within the cell 140. In another example, the pump optical signal 130 and the probe optical signal 135 are introduced into the vapor cell 140 at opposite ends of the vapor cell 140 but along the same axis so that their optical paths are aligned.When the wavelengths of these optical signals 130 and 135 are not at a peak in the vapor absorption spectrum, little or none of the modulation in the pump optical signal 130 is transferred to the probe optical signal 135. However, when the wavelengths of these optical signals 130 and 135 are close to a peak in the absorption spectrum, the modulation is transferred from the pump optical signal 130 to the probe optical signal 135, which is then detected by a control system 150 using an optical signal(s) 145 coming out of the vapor cell 140. The control system 150 (e.g., hardware (e.g., electronics, ASIC, FPGA, etc.), software, or combinations thereof) can lock the optical source 105 to the wavelength corresponding to a peak in the absorption spectrum of the vapor cell 140 or the fluorescence of the vapor cell 140. The optical source 105 can then be used in precise timing applications such as atomic clocks as well as other applications. The control system 150 uses the optical output signals 145 to generate a wavelength adjustment signal 160 to adjust the optical source 105. For example, the control system 150 can sweep the carrier frequency or wavelength of the unmodulated signal 110 until it coincides with a peak in the vapor absorption spectrum in cell 140. The control system 150 can then continue monitoring the output signals 145 to keep the carrier frequency or wavelength of signal 110 locked to the frequency corresponding to the peak in the absorption or fluorescence spectrum using the wavelength adjustment signal 160. Therefore, as environmental conditions change (e.g., changes in temperature, humidity, etc.)) or as the optical source ages, the spectroscopy system 100 can maintain the optical source by emitting a signal at the desired wavelength or frequency that coincides with a peak in the absorption spectrum of the vapor cell 140. In addition to the wavelength adjustment signal 160, the control system 150 outputs an AC control signal 155 to the AOM 120 to perform amplitude modulation to counteract the unintentional RAM acquired by the pump beam as it moves from the AOM to the vapor cell. Furthermore, in one embodiment, the AC control signal 155 can perform frequency modulation on the pump optical signal. Additionally, in one embodiment, the AC control signal 155 can control the AOM 120 to establish a fixed frequency offset between the pump and probe optical signals 130 and 135 (e.g., 200 MHz). The control system 150 also emits a power control signal 165 to adjust the VOA 125. For example, the control system 150 can use photodiodes to detect when the power in the upper and lower optical paths is not in the desired ratio and adjust the VOA 125 accordingly. The control system 150 additionally emits a power control signal 170 that controls the total power of the optical signals in system 100. For example, the power control signal 170 can adjust the output power of a laser in the optical source 105, thereby affecting the power of the optical signals from both pump and probe 130, 135. Therefore, the optical source 105 can control the total DC power of the optical signals in system 100, while the VOA 125 establishes the power ratio between the optical signals from pump and probe 130, 135. Figure 1B is similar to Figure 1A except that two different optical sources are used to generate the pump and probe optical signals 130 and 135. That is, optical source 105 generates the pump optical signal 130, while optical source 190 sends an optical signal to VOA 125 to generate the probe optical signal 135. However, while Figure 1B illustrates a separate VOA 125, in another embodiment an attenuator is integrated into optical source 190. That is, optical source 190 may include a built-in amplifier (such as the semiconductor optical amplifier 415 shown in Figure 4). In this example, optical source 190 is used for probe power control, eliminating the need for the VOA. The optical source 190 can be a single laser or a laser system. For example, the optical source 190 can represent a system of components, as described in Figures 4A and 4B below. In one embodiment, the optical source 190 includes a frequency converter, such as an SHG module, that adjusts the wavelength of the optical signal generated by the source 190. In addition, a separate wavelength adjustment signal 172 is sent to the optical source 190. In one embodiment, the wavelengths of the optical sources 105 and 190 can be synchronized (e.g., at a fixed offset). Furthermore, a separate AOM 120 may not be required in the system if the optical source 105 includes a built-in modulator, which is discussed in Figure 1C. That is, the laser for the optical source 105 can include a modulator function to generate the pumping optical signal 130 directly, without requiring the AOM 120. Figure 1C shows an embodiment in which two different optical sources are used to generate the pump and probe optical signals 130 and 135. Specifically, optical source 105 generates the pump optical signal 130, while optical source 190 emits a probe optical signal 135. The use of two optical sources 105 and 190 allows for a laser system with fewer discrete modulators and power control elements, which can be advantageous for reducing power, size, and cost. Specifically, optical sources 105 and 190 can include a built-in amplifier (such as the semiconductor optical amplifier 415 shown in Figure 4) or other power control function. The pumping optical source 105 may include modulation capabilities for frequency or phase modulation control in response to the AC control signal 185 and amplitude modulation in response to the AC control signal 157 since no light from source 105 is used to generate the probe optical signal 135. The optical source 190 can be a single laser or a laser system. For example, the optical source 190 can represent a system of components, as described in Figures 4A-4B below. In one embodiment, the optical source 190 includes a frequency converter, such as an SHG module, that adjusts the wavelength of the optical signal generated by the source 190. In another embodiment, the optical source 190 consists of a single laser element, such as a diode-pumped solid-state (DPSS) laser, which includes intracavitary frequency doubling and allows control of both wavelength and power. In yet another embodiment, the optical source 190 consists of a laser operating at the target wavelength (such as a 532 nm laser diode) and a VOA for power control. In one embodiment, the wavelengths of the optical sources 105 and 190 can be synchronized so that they emit optical signals at a fixed frequency offset. In Figure 1C, a separate AOM is not required in the system because the optical source 105 can include a built-in modulator. Figure 4B shows an embodiment for a source 105 with a built-in modulator, which will be discussed next. Figure 2 illustrates a spectroscopy system 200, according to one embodiment. System 200 is the same as System 100 in Figure 1A except for the addition of an optical isolator (ISO) 205 between the splitter 115 and the AOM 120. In one embodiment, the ISO 205 prevents back-reflected light from the AOM 120 and the vapor cell supply path from reaching the splitter 115 and the VOA 125. Without the ISO 205, the frequency and / or amplitude modulation performed by the AOM 120 could inadvertently combine with the optical probe signal 135.This parasitic modulation in the probe optical signal 135 is undesirable because it can result in the control system 150 mistakenly identifying the modulation in the probe optical signal 135 as transferred to this signal in the vapor cell 140 at a peak in the absorption spectrum when, in fact, it was inadvertently transferred to the probe optical signal 135 before the signal was transmitted through the vapor cell 140. ISO 205 can prevent (or mitigate) any parasitic modulation introduced into the pump optical signal 130 from being transferred to the probe optical signal 135 before these signals are introduced into the vapor cell 140. In another embodiment, a third optical signal (for example, also referred to as a reference optical signal) can be used to compensate for this parasitic modulation. The third optical signal can be an unmodulated reference optical signal (such as probe optical signal 135) that also passes through the vapor cell, but unlike probe optical signal 135, it is neither aligned with nor overlaps the pump optical signal 130. The third optical signal can be generated from the same optical signal as probe optical signal 135 using a divider. Therefore, any parasitic modulation in probe optical signal 135 due to AOM 120 will also appear in the third optical signal. The control system 150 can then use the third optical signal to subtract the parasitic modulation from probe optical signal 135.The third optical signal could be used in conjunction with ISO 205 to improve the system's ability to eliminate parasitic modulation (or more generally, for noise mitigation), or it could be used in the embodiments in Figures 1A and 1B where there is no ISO 205. Figure 5 shows an embodiment of the third optical signal 506 generated after VOA 125 and transmitted through the vapor cell before it is detected by photodiode 505E. In any case, the spectroscopy systems 100 and 200 in Figures 1 and 2 can be applied to derivative-based MTS or FMS spectroscopy. Figure 3 is a flowchart of a Method 300 for preparing optical signals for spectroscopy, according to one embodiment. Method 300 can be used to perform MTS or derivative-based FMS spectroscopy, but is not limited to such applications. In block 305, the spectroscopy system receives an optical signal. In one embodiment, the optical signal is an unmodulated optical signal (e.g., unmodulated optical signal 110 in Figures 1 and 2). The optical signal may have been generated using a laser source and a frequency converter (e.g., an SHG module) that changes the laser's wavelength and frequency to match the absorption spectrum of a vapor cell. For example, the laser source may emit an optical signal with a wavelength of 1064 nm, which is then reduced by an SHG module to generate an optical signal of 532 nm, within the range of a particular material in the vapor cell. In another example, the laser source may emit an optical signal with a wavelength of 1560 nm, which is then reduced by an SHG module to generate an optical signal of 780 nm, within the range of a particular material in the vapor cell.Of course, in other embodiments, the laser source can emit an optical signal that is already within the range of the vapor cell (e.g., 532 nm, 780 nm or 1542 nm) in which case the SHG module can be omitted. The embodiments described herein may include a frequency converter that modifies the wavelength generated by the laser source. For example, instead of using an SHG module, a frequency converter can be used that reduces the wavelength by one-third or one-quarter. In other words, the frequency converter can change the wavelength of the optical signal generated by the source so that it is an integer multiple of the wavelength of the optical signal emitted by the frequency converter. In block 310, the spectroscopy system splits the optical signal into two optical paths. A splitter can use any desired ratio to divide the power in the received optical signal between the two optical paths, for example, 50 / 50, 60 / 40, etc. In block 315, the spectroscopy system generates an optical pump signal by performing frequency and amplitude modulation on an initial portion of the optical signal using an AOM (Amplitude Modulator). In other words, an AOM is positioned in one of the two optical paths. This AOM can perform both frequency and amplitude modulation in parallel to generate the optical pump signal. The AOM also provides a frequency shift between the optical pump and probe signals. As discussed earlier, frequency modulation can add a preset oscillation to the optical pump signal, which can help identify a peak in the absorption spectrum. That is, in addition to sweeping (or tuning) a frequency of the optical signal using the laser source, the AOM can introduce frequency modulation to perform blurring within a defined frequency range. Amplitude modulation can be used to eliminate unwanted amplitude modulation acquired by the pumping beam. As discussed in more detail below, the amplitude can be opposite in phase with the amplitude modulation already present in the optical signal to eliminate the unwanted modulation. In block 320, the spectroscopy system generates a probe optical signal by adjusting the power of a second portion of the optical signal in a different optical path. In one embodiment, this optical path includes a VOA that adjusts the power of the probe optical signal to achieve a desired power ratio between the probe and pump optical signals in the two optical paths. However, in other embodiments, the VOA may be arranged in the pump optical signal path. In block 325, the spectroscopy system transmits the pump and probe optical signals to a vapor cell for spectroscopy. For example, the vapor cell may contain iodine, acetylene, or rubidium. One or more optical signals emanating from the vapor cell can be detected and used to generate timing signals to lock the laser source to a particular frequency. This locked signal can then be used in high-precision applications such as atomic sensors or clocks. Figure 4A illustrates an optical source 105 for performing spectroscopy, according to one embodiment. That is, Figure 4A illustrates various components that may be in the optical source 105 that generate the unmodulated optical signal 110, either in the single-source embodiment shown in Figure 1A or in the two-source embodiments shown in Figures 1B and 1C. The source 105 includes a seed optical source 405 (for example, a laser) that emits an optical signal with a first wavelength. In one embodiment, the seed optical source 405 is a low-power semiconductor seed laser. In another embodiment, the seed optical source 405 is a fiber laser. In yet another embodiment, the seed optical source 405 is a master oscillator power amplifier (MOPA) laser. Source 105 also includes an optical isolator 410 that prevents back reflections from reaching the seed optical source 405. In addition, a semiconductor optical amplifier (SOA) 415 amplifies the optical signal generated by the seed optical source 405. However, other types of optical amplifiers, such as a fiber amplifier, can be used, but these amplifiers may not provide the same power and modulation benefits as the SOA 415 and may be larger. A frequency converter 420 adjusts the wavelength (and frequency) of the optical signal generated by the seed optical source 405 to a different wavelength suitable for spectroscopy. For example, optical transitions at 532 nm and 514.7 nm have been well studied due to their narrow linewidths and the fact that these wavelengths are accessible by SHGs of neodymium- and ytterbium- (or erbium)-based lasers, respectively. To eliminate first-order Doppler effects associated with interrogating hot atomic vapor, laser systems can employ MTS or FMS. Advantageously, by adjusting the wavelength / frequency of the optical signal at the optical source 105, only one frequency converter 420 is used. In contrast, other spectroscopy implementations may place SHG modules in both the pump and probe optical signal paths (e.g., before these signals enter the vapor cell). However, in this embodiment, the systems in Figures 1A and 1B do not need to include SHG modules (or any other type of frequency converter) between the AOM 120 and the vapor cell 140 or between the VOA 125 and the vapor cell 140. The frequency converter 420 can adjust the wavelength to half, a third, etc., depending on the application. Therefore, the frequency of the optical signal 110 can be any integer multiple of the frequency of the optical signal generated by the seed optical source 405. For example, an SHG module, such as the frequency converter 420, can be used to convert a 1064 nm optical signal into a 532 nm optical signal for MTS or FMS spectroscopy. Therefore, the optical signal 110 has a different wavelength than the optical signal generated by the seed optical source 405. In another embodiment, the seed optical source 105 is a diode-pumped solid-state (DPSS) laser such as Nd:YV04 or Yb:KGW. In that case, the DPSS laser performs the combined functions of the seed optical source 405, the SOA 415, and the frequency converter 420. Therefore, in one embodiment, the four separate optical components in Figure 4A can be replaced by a single optical component that generates the unmodulated optical signal 110. Figure 4B illustrates an optical source 105 for performing spectroscopy, according to one embodiment. In one embodiment, Figure 4B is used in the embodiments shown in Figure 1C where the pump and probe optical signals are generated using two different optical sources 105, 190. The 405 seed optical source can perform wavelength adjustment and frequency modulation in response to the 160 wavelength adjustment signal and the 185 AC control signal. SOA 415 performs power control in response to power control signal 170. Additionally, SOA 415 can also be used to mitigate RAM in the pump signal by using AC control signal 157 to perform amplitude modulation of the SOA current. This suppresses unwanted amplitude modulation in the seed laser generated by source 405 or other elements of the laser system. Therefore, in this example, AC control signal 185 performs frequency modulation on the seed optical source 405, while AC control signal 157 performs amplitude modulation on SOA 415. This allows the AOM to be omitted from the system, as shown in Figure 1C, since SOA 415 can perform amplitude modulation. Furthermore, the VOA can be omitted from the system, as shown in Figure 1C. The isolator 410 and the frequency adjuster 420 can function the same as in Figure 4A. In another embodiment, an EOM could be used in the optical source 105 to provide the frequency or phase modulation required for the pumping beam 130. Figure 5 illustrates a spectroscopy system 500, according to one embodiment. The spectroscopy system 500 includes the optical source 105, ISO 205, AOM 120, VOA 125, and a spectroscopy subsystem 525, which may include, for example, a vapor cell. These components were described earlier and will not be described in detail here. The spectroscopy subsystem 525 includes several photodetectors (PDs) 505 for detecting optical signals at intermediate points in the system 500, as well as optical signals exiting the vapor cell. The electrical signals generated by the PDs 505 are then supplied to an IC 510 (for example, an ASIC or FPGA) that is part of the control system 150. The IC 510 then outputs the control signals 155, 160, 165, and 170 introduced in Figures 1A and 1B. In other words, IC 510 illustrates logic (e.g., hardware circuitry) to generate these signals using the PD 505 outputs. In this embodiment, the PD 505A and the AOM 515 synthesizer are used to generate an amplitude modulation signal (e.g., the control signal of AC 155) to compensate for RAM or any other unwanted amplitude modulation in the optical pump signal generated by the AOM 120. For example, the optical path including the AOM 120 can split a small portion of the optical pump signal to the PD 505A before the optical pump signal passes through the vapor cell. The split can be polarization-insensitive to the optical pump signal to eliminate polarization-dependent variations in the optics or fiber used for delivery. The electrical output of the PD 505A (e.g., an AC component of the optical pump signal) can then be used by the IC 510 as part of the control signal of AC 155 to mitigate or eliminate unwanted amplitude modulation in the optical pump signal. To do this, the output of the PD 505A is connected to a RAM AC circuit (e.g., an analog-to-digital converter (ADC)). The output of the RAM AC circuit is connected to a RAM I / Q demodulator (demod) and I and Q servos. The outputs of these servos generate AI and AQ feedback signals that are used as inputs to an AOM 515 synthesizer (e.g., a digital synthesizer) which generates the AC 155 control signal. In addition to mitigating unwanted amplitude modulation, the AC 155 control signal can also generate a frequency offset (e.g., 200 MHz) between the pump optical signal and the probe optical signal. Therefore, in this embodiment, the AOM 515 synthesizer can use the equation shown in Figure 5 to generate signal 155, which performs three tasks: it mitigates amplitude modulation in the pump optical signal, it performs frequency modulation on the pump optical signal, and it establishes a frequency offset of the pump beam relative to the probe beam. In another embodiment (e.g., for the embodiments shown in Figures 4B and 1C), two AC control signals can be generated to separately output the AC 185 control signal and the AC 157 control signal. In addition to receiving inputs from the PD 505A, the AOM 515 synthesizer also receives parameter inputs for Ac, c, β, and modulo, providing it with the data necessary to perform frequency modulation and establish the frequency shift. Figure 5 illustrates an implementation of a control system for the optical pump signal that uses digital synthesis, as opposed to analog circuitry. This approach can improve signal quality, eliminate intermodulation mixing products, reduce the potential crosstalk of modulating multiple devices at the same baseband frequency, and eliminate any analog mixing circuitry. For example, by using a digital AOM 515 synthesizer, this synthesis process does not rely on analog mixers that might add unwanted tones to the signal.However, the circuitry shown in IC 510 is only one suitable implementation of a digital synthesis technique for controlling the AOM 120. In another embodiment, square wave modulation can be used for both frequency and amplitude modulation. The PD 505B generates an MTS signal for performing MTS spectroscopy. For example, the PD 505B can detect the optical probe signal after it has passed through the vapor cell. The electrical output of PD 505B (e.g., an AC component of the probe optical signal) is received by an MTS AC circuit (e.g., an ADC) in IC 510. IC 510 includes digital circuitry (e.g., MTS demodulation I / Q, Q monitoring circuit, servo I, and DAC) to generate the wavelength adjustment signal 160, which adjusts the wavelength of the optical signal generated by optical source 105. Although Figure 5 illustrates the implementation of MTS spectroscopy, as discussed above, the left half of the 500 spectroscopy system (i.e., optical source 105, ISO 205, AOM 120, and VOA 125) can also be used to perform other types of spectroscopy, such as FMS. In that case, the digital circuitry in IC 510 may be different. The PD 505C generates a pump power signal that adjusts the total power of the optical signals in the 500 system. For example, a divider can be used to divert a small portion of the pump optical signal to the PD 505C before the pump optical signal passes through the vapor cell 140. The electrical output of the PD 505C (for example, a DC component of the pump optical signal) is received in a pump DC circuit in IC 510, which includes a DC power servo and a Pump_Pwr DAC, to output the power control signal 170 to the source optic 105. The PD 505D generates a probe power signal that adjusts the power of the optical probe signal. For example, a splitter can be used to divert a small portion of the optical probe signal to the PD 505D before the optical probe signal passes through the vapor cell 140. Another implementation places the PD 505D after the vapor cell 140 where the optical probe signal can be detected. The electrical output of the PD 505D (e.g., a DC component of the optical probe signal) is received by a probe DC circuit in IC 510, which includes a DC power servo and a Probe_Pwr DAC, to output the power control signal 165 to the VOA 125. As discussed earlier, the power control signal 165 can adjust the VOA 125 to establish a desired power ratio between the optical pump and probe signals. The implementation of the power servos may differ depending on the location of the VOA (e.g., in the pump or probe optical paths) or whether an optical source has a built-in attenuator. For example, if the VOA is integrated into the optical source (e.g., SOA 415 in Figure 4), the power control signal 165 can be fed directly to the optical source. The PD 505E generates a probe reference signal for detection balanced with the MTS signal generated by the PD 505B. For example, the PD 505E can be used when the optical source 105 has significant residual intensity noise (RIN), such as DPSS lasers. Additionally, the PD 505E can be used when the optical source 105 has significant frequency noise. The PD 505E can also mitigate the impact of unwanted leakage of the modulated pump light on the probe signal. However, the PD 505E can be omitted from the system. In one embodiment, the PD 505E is a fiber-coupled photodiode that receives the optical signal 506. In another embodiment, the optical signal 506 is generated with free-space optics within the spectroscopy subsystem prior to measurement with the PD 505E. In one embodiment, the optical components in the 500 system (e.g., optical source 105, ISO 205, AOM 120, and VOA 125) are fiber-coupled. In one embodiment, the AOM 120 also includes a 520 polarizer that mitigates or eliminates differential RAM in possible orthogonal light polarizations. The 520 polarizer can be arranged at the input or output of the AOM 120. While Figure 5 illustrates the use of four PDs (i.e., PDs 505A-D) to detect AC and DC components from the pump and optical probe signals before or after these signals pass through the vapor cell in the spectroscopy subsystem 525, in another embodiment, the system 500 can have only two PDs that detect the pump and optical probe signals, respectively. These two electrical signals can then be processed in IC 510 and separated into their AC and DC components. For example, after IC 510 separates the electrical signal generated from the pump optical signal into an AC component and a DC component, the AC component can be transmitted to the RAM AC output, and the DC component can be transmitted to the pump DC output.After IC 510 separates the electrical signal generated from the optical probe signal into an AC component and a DC component, the AC component can be transmitted to the MTS AC and the DC component can be transmitted to the probe DC. The foregoing description is provided to enable any person skilled in the art to practice the various embodiments described herein. The examples discussed herein do not limit the scope, applicability, or embodiments set forth in the claims. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments. For example, changes may be made to the function and arrangement of elements discussed without departing from the scope of disclosure. Various examples may omit, substitute, or add various procedures or components as appropriate. For example, the methods described may be performed in a different order than described, and various steps may be added, omitted, or combined.Also, the features described with respect to some examples may be combined in some other examples. For example, an apparatus may be implemented or a method may be put into practice using any number of the aspects set forth herein. Furthermore, the scope of disclosure is intended to cover such an apparatus or method put into practice using a different structure, functionality, or structure and functionality in addition to, or different from, the various aspects of disclosure set forth herein. It should be understood that any aspect of disclosure disclosed herein may be incorporated by means of one or more elements of a claim. As used herein, the word "illustrative" means "serving as an example, instance, or illustration." Any aspect described herein as "illustrative" should not necessarily be interpreted as preferred or advantageous over other aspects. As used herein, a phrase referring to "at least one of" a list of elements refers to any combination of those elements, including individual members. For example, "at least one of: a, boc" is intended to cover a, b, c, ab, ac, bc, and abc, as well as any combination with multiples of the same element (e.g., aa, aaa, aab, aac, abb, acc, bb, bbb, bbc, cc, and ccc, or any other order of a, by, and c). As used herein, the term "determine" encompasses a wide variety of actions. For example, "determine" may include calculating, computing, processing, deriving, investigating, searching (e.g., looking up a table, database, or other data structure), and the like. "Determine" may also include receiving (e.g., receiving information), accessing (e.g., accessing data in memory), and the like. Additionally, "determine" may include resolving, selecting, choosing, setting, and the like. The methods disclosed herein comprise one or more steps or actions to achieve the desired results. The steps and / or actions of the method may be interchanged without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and / or use of specific steps and / or actions may be modified without departing from the scope of the claims. Furthermore, the various operations of the methods described above may be performed by any suitable means capable of performing the corresponding functions. The means may include various hardware and / or software components and / or modules, including, but not limited to, a circuit, an application-specific integrated circuit (ASIC), or a processor. In general, where operations are illustrated in the figures, those operations may have corresponding equivalent means and function components with similar numbering. As a person skilled in the art will appreciate, the embodiments disclosed herein may be realized as a system, method, or software product. Accordingly, the embodiments may take the form of a purely hardware embodiment, a purely software embodiment (including firmware, resident software, microcode, etc.), or an embodiment that combines software and hardware aspects, which may be referred to herein as a "circuit," "module," or "system." Furthermore, the embodiments may take the form of a software product embedded in one or more computer-readable media containing computer-readable program code. Program code embedded in a computer-readable medium may be transmitted using any appropriate means, including, but not limited to, wireless, wired, fiber optic cable, RF, etc., or any suitable combination thereof. The computer program code to perform operations for the realizations described in this disclosure may be written in any combination of one or more programming languages, including an object-oriented programming language such as Java, Smalltalk, C++, or similar languages, and conventional procedural programming languages ​​such as the "C" programming language or similar programming languages. The program code may run entirely on the user's computer, partly on the user's computer as a standalone software package, partly on the user's computer and partly on a remote computer, or entirely on the remote computer or server.In the last scenario, the remote computer can connect to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection can be made to an external computer (for example, through the Internet using an Internet service provider). The aspects of this disclosure are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and software products in accordance with embodiments presented in this disclosure. It is understood that each block in the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by software instructions.These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which are executed through the processor of the computer or other programmable data processing apparatus, create means to implement the functions / acts specified in the block or blocks of the flowchart illustrations and / or block diagrams. These computer program instructions can also be stored on a computer-readable medium that can direct a computer, other programmable data processing apparatus, or other device to operate in a particular manner, such that the instructions stored on the computer-readable medium produce a manufactured item, including instructions that implement the function / action specified in the block or blocks of the flowchart illustrations and / or block diagrams. Computer program instructions can also be loaded into a computer, other programmable data processing device, or other device to cause a series of operational steps to be performed in the computer, other programmable device, or other device to produce a computer-implemented process such that the instructions executed in the computer, other programmable data processing device, or other device provide processes to implement the functions / acts specified in the block or blocks of the flowchart illustrations and / or block diagrams. The flowchart and block diagram illustrations in the figures depict the architecture, functionality, and operation of possible implementations of computer programs, methods, and products according to various embodiments. In this context, each block in the flowchart or block diagram illustrations may represent a module, segment, or portion of code, comprising one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions indicated in the block may occur outside the order shown in the figures. For example, two blocks shown sequentially may, in fact, execute substantially concurrently, or the blocks may sometimes execute in reverse order, depending on the functionality involved.It will also be noted that each block in block diagrams and / or flowchart illustrations, and combinations of blocks in block diagrams and / or flowchart illustrations, can be implemented by special-purpose hardware-based systems that perform the specified functions or acts, or combinations of special-purpose hardware and computer instructions.

Claims

1. A laser system for performing spectroscopy, comprising: an optical source (105) configured to generate a first optical signal; a splitter (115) configured to split the first optical signal into a second optical signal and a third optical signal; an acousto-optic modulator, AOM, (120) configured to modulate the second optical signal to emit a pump optical signal (130); an optical isolator (205) disposed between the splitter and the AOM configured to prevent return reflections from the AOM from reaching the splitter; a variable optical attenuator, VOA, (125) configured to adjust the power of either the second optical signal or the third optical signal, wherein the third optical signal is used to generate a probe optical signal (135); and a vapor cell (140) configured to receive the pump optical signal and the probe optical signal.

2. The system of claim 1,wherein the AOM is configured to perform frequency modulation to oscillate the pump optical signal and, especially, wherein the AOM is configured to perform amplitude modulation on the pump optical signal in parallel with performing the frequency modulation, wherein the amplitude modulation mitigates or eliminates unwanted amplitude modulation caused by residual amplitude modulation (RAM).

3. The system of claim 1, further comprising: a splitter configured to split the probe optical signal to emit a reference optical signal that passes through the vapor cell but does not overlap with the pump optical signal in the vapor cell, wherein the reference optical signal is used for noise mitigation; or wherein the AOM includes a polarizer configured to mitigate differential RAM in the pump optical signal.

4. The system of claim 1,wherein the optical source comprises: a seed optical source configured to generate a seed optical signal; a semiconductor optical amplifier (SOA) or a fiber amplifier configured to receive the seed optical signal and output an amplified optical signal; and a frequency converter configured to change a wavelength of the amplified optical signal to generate the first optical signal.

5. The system of claim 4, wherein a wavelength of the seed optical signal is an integer multiple of a wavelength of the first optical signal.

6. The system of claim 4, further comprising: a control system configured to generate: a wavelength adjustment signal for the seed optical source to change the wavelength of the seed optical signal based on the reception of an output optical signal from the vapor cell,A power control signal for the SOA or fiber amplifier to control the output power of the pump optical signal, an alternating current (AC) control signal for the AOM to modulate the pump optical signal, and a power signal to the VOA to attenuate the power of either the second or third optical signal.

7. The system of claim 1, wherein there are no frequency converters arranged between the AOM and the vapor cell or between the VOA and the vapor cell; or wherein there is no electro-optical modulator arranged between the AOM and the vapor cell.

8. A method for performing spectroscopy, comprising: receiving a first optical signal; splitting the first optical signal into a second and a third optical signal; generating, using an AOM,an optical pump signal modulating both the frequency and amplitude of the second optical signal; transmitting the second optical signal through an optical isolator before reaching the AOM, wherein the optical isolator prevents return reflections from the AOM from passing through it; generating an optical probe signal using the third optical signal, wherein a VOA adjusts the power of either the second or the third optical signal; and transmitting the optical pump and probe signals to a vapor cell for spectroscopy.

9. The method of claim 8, further comprising: transmitting a power control signal to an optical source that generates the first optical signal, wherein the power control signal adjusts the power of both the optical pump signal and the optical probe signal.

10. The method of claim 8,wherein providing the first optical signal comprises: generating a seed optical signal; amplifying the seed optical signal using a SOA; and adjusting a wavelength of the seed optical signal.

11. The method of claim 10, wherein a wavelength of the seed optical signal is an integer multiple of a wavelength of the first optical signal; or further comprising: receiving an output optical signal from the vapor cell in a photodiode to generate a spectroscopy signal; and generating, based on the spectroscopy signal, a wavelength adjustment signal to change a wavelength of the seed optical signal.

12. A laser system for performing spectroscopy,comprising: a first optical source (105) configured to generate a first optical signal; a second optical source (190) configured to generate a second optical signal; a first power control signal path (165) configured to adjust the power of the second optical signal to generate a probe optical signal (135); an AC control signal path (185) configured to frequency modulate the first optical signal; and a second power control signal path (170) to control the power of the first optical signal to emit a pump optical signal (130); and a vapor cell (140) configured to receive the pump optical signal and the probe optical signal.

13. The system of claim 12, wherein the first optical source comprises a semiconductor optical amplifier, SOA,configured to receive an AC amplitude control signal that modulates the SOA to perform amplitude modulation on the first optical signal; particularly wherein the spectroscopy system does not include an AOM, or wherein the first optical source comprises a seed optical source configured to receive an AC control signal to frequency modulate the first optical signal.

14. The system of claim 12, further comprising: a VOA that is separate from the second optical source and receives a first power control signal, wherein the VOA is configured to emit the probe optical signal; or further comprising: an AOM that is separate from the first optical source, wherein the AOM is configured to receive an AC amplitude control signal that modulates the AOM to perform at least one amplitude modulation on the first optical signal.

15. An integrated circuit (510) included in the laser system according to claims 1 to 7,comprising: a digital synthesizer (515) configured to: generate a frequency-modulated control signal for an optical pump signal, wherein the optical pump signal is used in vapor cell spectroscopy, and generate an amplitude modulation signal to compensate for unwanted amplitude modulation in the optical pump signal; a first digital servo configured to, based on an AC component of an optical probe signal, control the wavelength of an optical source that generates at least one of the optical pump signal or the optical probe signal, wherein the optical probe signal is used in vapor cell spectroscopy; a second digital servo configured to, based on an AC component of the optical pump signal, control amplitude modulation parameters in the digital synthesizer; a third digital servo configured to, based on a DC component of the optical pump signal,emit a first power control signal to adjust the power of at least one of the optical pump or probe signals; and a fourth digital servo configured to, based on a DC component of the optical probe signal, emit a second power control signal to adjust the power of at least one of the optical pump or probe signals.

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