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EP-4369535-B1 - LASER SYSTEMS FOR SPECTROSCOPY

EP4369535B1EP 4369535 B1EP4369535 B1EP 4369535B1EP-4369535-B1

Inventors

  • ROSLUND, JONATHAN DAVID
  • BOYD, MARTIN MACHAI
  • CINGOZ, ARMAN

Dates

Publication Date
20260506
Application Date
20231005

Claims (15)

  1. A laser system to perform 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 output a pump optical signal (130); an optical isolator (205) disposed between the splitter and the AOM configured to prevent back reflections from the AOM from reaching the splitter; a variable optical attenuator, VOA, (125) configured to adjust the power of one of 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 a frequency modulation to dither the pump optical signal, and especially, wherein the AOM is configured to perform an amplitude modulation on the pump optical signal in parallel with performing the frequency modulation, wherein the amplitude modulation mitigates or removes an undesired 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 output 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 receiving an output optical signal from the vapor cell, a power control signal for the SOA or the fiber amplifier to control an 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 for attenuating the power of one of the second optical signal or the third optical signal.
  7. The system of claim 1, wherein there are no frequency converters disposed between the AOM and the vapor cell or between the VOA and the vapor cell; or wherein there is no electro-optic modulator disposed between the AOM and the vapor cell.
  8. A method to perform spectroscopy, comprising: receiving a first optical signal; splitting the first optical signal into a second optical signal and a third optical signal; generating, using an AOM, a pump optical signal by performing both frequency and amplitude modulation on the second optical signal; transmitting the second optical signal through an optical isolator before reaching the AOM, wherein the optical isolator prevents back reflections from the AOM from passing therethrough; generating a probe optical signal using the third optical signal, wherein a VOA adjusts the power of one of the second optical signal or the third optical signal; and transmitting the pump and probe optical signals into a vapor cell to perform spectroscopy.
  9. The method of claim 8, further comprising: transmitting a power control signal to an optical source generating the first optical signal, wherein the power control signal adjusts the power of both the pump optical signal and the probe optical 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 at 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 to perform 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 output 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; especially 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 output 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 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 a pump optical signal, wherein the pump optical signal is used in vapor cell spectroscopy, and generate an amplitude modulation signal for compensating for undesired amplitude modulation in the pump optical signal; a first digital servo configured to, based on an AC component of a probe optical signal, control the wavelength of an optical source that generates at least one of the pump optical signal or the probe optical signal, wherein the probe optical signal is used in vapor cell spectroscopy; a second digital servo configured to, based on an AC component of the pump optical signal, control amplitude modulation parameters in the digital synthesizer; a third digital servo configured to, based on a DC component of the pump optical signal, output a first power control signal to adjust the power of at least one of the pump or probe optical signals; and a fourth digital servo configured to, based on a DC component of the probe optical signal, output a second power control signal to adjust the power of at least one of the pump or probe optical signals.

Description

BACKGROUND Field Embodiments of the present invention generally relate to a laser system to perform spectroscopy. Description of the Related Art Optical atomic clocks offer improved frequency instabilities compared to microwave frequency standards due to the higher quality factor Q associated with an optical resonance. In order to take advantage of these high quality factors, a coherent interaction between the light and matter is required. One barrier to the widespread deployment of optical frequency standards is the requirement to develop compact, robust, and low-power laser sources amenable to integration at the optical frequency of interest. Additionally, the laser system must offer a means for controlling the systematic errors typically associated with optical frequency standards, including residual amplitude modulation (RAM) and AC-light shifts. In order to eliminate first-order Doppler effects associated with interrogating a warm atomic vapor, the laser systems typically employ modulation transfer spectroscopy (MTS) or frequency modulation spectroscopy (FMS). However, these approaches often rely on multiple expensive and power hungry optical components such as acousto-optic modulators (AOM) and Electro-optic modulators (EOMs). For example, previously demonstrated MTS techniques start with two distinct optical beams, pump and probe signals, where each of these beams passes through its own AOM. The pump beam AOM creates both a static frequency offset from the probe and imparts a modulation. Often the laser system requires frequency doubling to reach the sample wavelength, which adds complexity to the generation of the pump and probe beams with appropriate characteristics. Each of the beams may pass through a respective second harmonic generation (SHG) module to change the wavelength of the signals. MTS spectroscopy is performed with the two resultant beams. However, providing AOMs and SHG modules in both optical paths adds substantial cost to the system, bulk, and power consumption. Generation of the two spectroscopy beams with appropriate characteristics for MTS is challenging in a simple laser system due to requirements that the probe beam have no spurious modulation present from the pump light or control electronics. In addition, reliable reduction of RAM in the pump beam is complicated by polarization properties of modulators. Fiber delivery of the beams to the atomic setup is ideal in many cases, however fiber exacerbates potential for undesired modulated light on the probe beam path through etalons, and improper control of RAM through polarization variation over temperature and time in the fibers. Document "OPTICAL FREQUENCY STANDARD AT 532 NM" by Eickhoff M. L. et al. discloses frequency-doubled Nd:YAG laser source. Document "Semiconductor laser active frequency Stabilization technologies: a review" by Wu Yue et al. discloses the basic principles, and features and the latest research progress on various kinds of semiconductor laser, active frequency stabilization techniques based on different locking reference standards. Document "DOPPLER-FREE TWO-PHOTON MODULATION TRANSFER SPECTROSCOPY IN SODIUM DIMERS" by Ma L. S. et al. reports on modulation transfer spectroscopy in two-photon transition of NA2 using a CW dye laser. Document "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 apparatuses to stabilize two independent lasers. US 4 590 597 A discloses how to precisely stabilize a laser to a sud-Doppler resonance. SUMMARY The above-mentioned problems are overcome with the features of the independent claims. BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments. 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 performing 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 numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRI