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EP-4738627-A1 - OFFSET LOCKED DUAL LASER SYSTEM FOR MTS SPECTROSCOPY

EP4738627A1EP 4738627 A1EP4738627 A1EP 4738627A1EP-4738627-A1

Abstract

Embodiments herein describe photonic systems with two laser sources that are tasked with generating the pump and probe optical signals for MTS spectroscopy. However, instead of having the same frequency, a control system can introduce a frequency offset which can be used to filter out interference between the optical signals that can occur before they pass through a vapor cell. In one embodiment, the optical signals are combined and detected at a photodiode that generates a beatnote that can be used to ensure the frequency offset is maintained.

Inventors

  • CINGOZ, ARMAN
  • BOYD, MARTIN MACHAI
  • KOWLIGY, ABIJITH SUDARSAN
  • ROSLUND, JONATHAN DAVID

Assignees

  • Vector Atomic, Inc.

Dates

Publication Date
20260506
Application Date
20251010

Claims (15)

  1. A photonic system, comprising: a first laser source configured to generate a pump optical signal for modulation transfer spectroscopy (MTS); a second laser source configured to generate a probe optical signal for MTS; an optical combiner configured to optically combine a portion of the pump and probe optical signals to generate a combined optical signal; a photodiode configured to receive the combined optical signal and output a beatnote; and a control system configured to, based on receiving the beatnote, adjust a parameter of at least one of the first or second laser sources to set a desired frequency offset between the pump and probe optical signals.
  2. The photonic system of claim 1, further comprising: a photonic integrated circuit (PIC) configured to receive the pump and probe optical signals, the PIC comprising: the combiner that optically combines a portion of the pump and probe optical signals; and amplitude controls configured to adjust amplitudes of the pump and probe optical signals.
  3. The photonic system of claim 2, further comprising: a phase modulator disposed on the PIC, or disposed on a second PIC that is optically coupled to the PIC, wherein the phase modulator is configured to receive a portion of the pump optical signal and modulate the pump optical signal for performing MTS.
  4. The photonic system of claim 3, wherein the control system is configured to control the phase modulator to generate spectroscopy signals derived from transmitting the pump and probe optical signals through a vapor cell.
  5. The photonic system of claim 3, wherein the phase modulator is disposed on the second PIC, wherein the second PIC is formed from a second order non-linearity material.
  6. The photonic system of claim 5, wherein the second PIC comprises frequency doublers configured to change the frequency of the pump and probe optical signals; especially wherein the second PIC comprises a Mach-Zehnder interferometer (MZI) structure configured to receive the pump optical signal, wherein a first part of the MZI structure implements a push-pull amplitude modulator to cancel out residual amplitude modulation while a second part of the MZI structure implements one of the amplitude controls.
  7. The photonic system of claim 3, wherein the phase modulator is disposed on the PIC, wherein the PIC includes at least a first layer that is a second order non-linearity material, wherein at least one of the amplitude controls is formed in a second layer of the PIC that does not include the second order non-linearity material; especially wherein the PIC comprises frequency doublers implemented using the first layer, wherein the frequency doublers are configured to change a frequency of the pump and probe optical signals.
  8. The photonic system of claim 2, wherein the PIC comprises resonator or filter structures to force single longitudinal mode operation and narrow a linewidth of the pump and probe optical signals before reaching the amplitude controls and the photodiode.
  9. The photonic system of claim 2, wherein the PIC is configured to receive a frequency comb and combine the frequency comb with a portion of the probe optical signal.
  10. The photonic system of claim 9, wherein the PIC comprises: a supercontinuum generation waveguide (SGW) configured to spread a spectrum of the frequency comb.
  11. The photonic system of claim 10, further comprising: a second PIC comprising a second order non-linearity material, the second order non-linearity material comprising a frequency doubler configured to change a frequency of the frequency comb after passing through the SGW
  12. The photonic system of claim 10, further comprising: a second PIC comprising a second order non-linearity material, wherein the second PIC is aligned with the PIC, wherein the second PIC comprises: a SGW configured to spread a spectrum of the frequency comb; and a frequency doubler formed using the second order non-linearity material, wherein the frequency doubler is configured to change the frequency of the frequency comb after passing through the SGW, wherein the frequency comb first passes through the second PIC before being optically combined with the portion of the probe optical signal in the PIC.
  13. The photonic system of claim 2, wherein the amplitude controls comprise Mach-Zehnder interferometers (MZI) comprising thermo-optic heaters as phase shifters in one arm.
  14. The photonic system of claim 1, wherein the control system is configured to modulate the first laser source in order to generate a modulated pump optical signal for performing MTS; and/or wherein the control system is further configured to use a spectroscopy signal resulting from passing the pump and probe optical signals through a vapor cell to lock the probe and pump optical signals.
  15. A method, comprising: receiving two laser signals from two separate laser sources; generating a beatnote between the two laser signals using a photodiode; setting a desired frequency offset between the two laser signals based on the beatnote; modulating one of the two laser signals to generate a pump optical signal, wherein the other of the two laser signals is unmodulated to generate a probe optical signal; and transmitting the pump optical signal and the probe optical signal to a vapor cell to perform modulation transfer spectroscopy (MTS).

Description

BACKGROUND Field Embodiments of the present invention generally relate to photonic systems 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 photonic 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 photonic 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 photonic 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 discrete 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 photonic 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. SUMMARY One embodiment described herein is a photonic system that includes a first laser source configured to generate a pump optical signal for modulation transfer spectroscopy (MTS), a second laser source configured to generate a probe optical signal for MTS, an optical combiner configured to optically combine a portion of the pump and probe optical signals to generate a combined optical signal, a photodiode configured to receive the combined optical signal and output a beatnote, and a control system configured to, based on receiving the beatnote, adjust a parameter of at least one of the first or second laser sources to set a desired frequency offset between the pump and probe optical signals. One embodiment described herein is a PIC that includes waveguides configured to direct a first portion of a pump optical signal to a vapor cell for MTS, combine a second portion of the pump optical signal with a first portion of a probe optical signal to generate a combined optical signal, direct the combined optical signal to a photodiode configured to output a beatnote for maintaining a frequency offset between the pump and probe optical signals, and direct a second portion of the probe optical signal to the vapor cell for MTS. One embodiment described herein is a method that includes receiving two laser signals from two separate laser sources, generating a beatnote between the two laser signals using a photodiode, setting a desired frequency offset between the two laser signals based on the beatnote, modulating one of the two laser signals to generate a pump optical signal, wherein the other of the two laser signals is unmodulated to generate a probe optical signal, and transmitting the pump optical signal and the probe optical signal to a vapor cell to perform modulation transfer spectroscopy (MTS). 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 equall