An optical cavity ring-down spectroscopy system based on dual optical path PDH mode locking technology

Abstract

The application discloses a kind of optical cavity ring-down spectroscopy systems based on double optical path PDH mode-locking technique, including laser frequency feedback control module, laser phase polarization modulation module and light splitting modulation module.Laser frequency feedback control module utilizes integrated laser frequency stabilizer to adjust the laser frequency output by external cavity diode laser.Laser phase polarization modulation module realizes polarization and phase modulation by Mach-Zehnder modulator and integrated laser frequency stabilizer.Light splitting modulation module realizes the generation of two polarized perpendicular beams by fiber beam splitter, half-wave plate, Faraday rotator and polarizer etc., and maintains the real-time locking of laser frequency.One is PDH light, for modulating laser emission frequency;Another is probe light, for actual measurement.Fast switch ensures that the input of probe light is cut off when ring-down event occurs, thereby reducing interference and improving signal-to-noise ratio.The application has higher speed and accuracy, improves the efficiency and reliability of spectral analysis.

Description

Technical Field

[0001] This invention relates to an optical cavity ring-down spectroscopy system based on dual-path PDH mode-locking technology, belonging to the field of laser spectroscopy. Background Technology

[0002] Cavity Ring-Down Spectroscopy (CRDS) is a high-precision, ultra-sensitive spectroscopic measurement technique used to study the light absorption and scattering processes of gaseous and liquid samples. The basic principle of CRDS is to obtain parameters such as the absorption or scattering coefficients of a sample by measuring the exponential decay of light intensity within the sample. In CRDS, light undergoes multiple reflections within the cavity, forming a back-and-forth propagating beam. When light passes through a sample or solution, it is absorbed or scattered, causing its intensity to decay over time. CRDS utilizes the decay rate of light intensity within the cavity over time to mathematically calculate the absorption or scattering characteristics of the sample.

[0003] In CRDS (Continuous Frequency Detection and Retention) technology, the frequency stability of the laser is crucial for the accuracy and repeatability of measurement results. Any frequency shift or fluctuation will introduce measurement errors. Traditional CRDS technology has certain limitations in terms of light source frequency stability. Summary of the Invention

[0004] This invention provides an optical cavity ring-down spectroscopy system based on dual-path PDH mode-locking technology, which solves the problems disclosed in the background art.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: an optical cavity ring-down spectral system based on dual-optical-path PDH mode-locking technology, characterized by: a laser frequency feedback control module, a laser phase polarization modulation module, and a beam splitting modulation module;

[0006] The laser frequency feedback control module includes an external cavity diode laser, a fiber coupler, and an integrated laser frequency stabilizer; the external cavity diode laser serves as a light source, emitting a laser of a specific frequency into the fiber coupler, which is used for fiber coupling of the laser.

[0007] The laser phase polarization modulation module includes a Mach-Zeder modulator. The laser, after being coupled by an optical fiber coupler, is delivered to the Mach-Zeder modulator and its phase and polarization are modulated under the integrated laser frequency stabilizer. The laser lock box of the integrated laser frequency stabilizer integrates a waveform generator, a mixer, a low-pass filter, and a dual-cascaded PID controller for PDH locking.

[0008] The beam splitting and modulation module includes an optical fiber splitter and a ring-down cavity. The modulated laser enters the optical fiber splitter, which splits the modulated laser into two beams, forming a PDH beam (solid line in the figure) and a probe beam (dashed line in the figure). After the PDH beam is injected into the ring-down cavity, it is reflected to form an optical signal. This optical signal is processed and converted by an integrated laser frequency stabilizer, and finally drives the external cavity diode laser through the generated electrical signal.

[0009] The probe light enters the ring-down cavity and is reflected countless times within the cavity to trigger a ring-down event.

[0010] Furthermore, the beam splitting modulation module also includes a first half-wave plate, which is used to rotate the polarization modulation of the PDH light by 180° so that it is perpendicular to the probe light.

[0011] Furthermore, the beam splitting and modulation module also includes a first polarization beam splitter, a second half-wave plate, a Faraday rotator, and a second polarization beam splitter; after the PDH light polarization is rotated by 180°, it is sequentially input into the attenuation cavity through the first polarization beam splitter, the second half-wave plate, the Faraday rotator, and the second polarization beam splitter.

[0012] Furthermore, it also includes a first photodetector. After the PDH light is input into the ring-down cavity, it is reflected to form an optical signal. The optical signal passes through a second polarization beam splitter, a Faraday rotator, a second half-wave plate, and a first polarization beam splitter in sequence, and is then reflected by a mirror into the first photodetector. The optical signal is received by the first photodetector, and then the first photodetector inputs the optical signal into the integrated laser frequency stabilizer.

[0013] Furthermore, the probe light is transmitted through the second polarization beam splitter into the ring-down cavity.

[0014] Furthermore, it also includes a fast switch and a second photodetector. When a ring-down event occurs, the fast switch immediately cuts off the incident probe light, and the second photodetector is used to detect the attenuation of the probe light in the cavity in real time to obtain the ring-down spectrum data of the optical cavity.

[0015] The beneficial effects achieved by this invention are as follows:

[0016] 1. This invention selects an ECDL external cavity diode laser as the light source. ECDL lasers are semiconductor lasers that utilize external cavity tuning technology to control the output light frequency. They possess narrow linewidth, high stability, and tuning performance, making them an ideal light source choice for CRDS technology. These characteristics enable ECDL lasers to provide a high-precision, reliable, and flexible light source for achieving accurate spectral analysis and measurement in CRDS technology, thus improving its accuracy and stability.

[0017] 2. In the PDH technology section of this invention, unlike common PDH technologies, a dual-optical-path PDH technology is used, which splits the PDH mode-locking light into two beams: a probe beam and a mode-locking beam. The mode-locking beam and the probe beam enter the ring-down cavity simultaneously with orthogonal polarization, thereby enabling rapid acquisition of the ring-down time without interrupting PDH locking.

[0018] 3. This invention uses a fiber optic Mach-Zehnder modulator (MZM) instead of an acousto-optic modulator. The MZM enables frequency shifting exceeding 1 GHz with millihertz-level accuracy. Furthermore, the drive RF signal of the MZM can be quickly switched off, triggering oscillation events within 20 nanoseconds. This represents a significant improvement over traditional acousto-optic modulators, whose poor attenuation ratio and switching bandwidth degrade the quality of oscillation measurements. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structured optical path of the present invention. Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0021] Glossary of terms: ECDL (External Cavity Diode Laser), MZM (Mach-Zeder Modulator), Integrated Laser Stabilizer

[0022] like Figure 1 As shown, this invention discloses an optical cavity ring-down spectroscopy system based on dual-path PDH mode-locking technology. It consists of three parts: a laser frequency feedback control module, a laser phase polarization modulation module, and a beam splitting modulation module. The main implementation is a frequency-locked detection system composed of an ECDL external cavity diode laser, an optical fiber coupler, an MZM Mach-Zeder modulator, an integrated laser frequency stabilizer, an optical fiber splitter, a half-wave plate, a Faraday rotator, a polarization beam splitter, a photodetector, and a ring-down cavity. The function of each device and their interrelationships will be described in detail below.

[0023] First, an ECDL external cavity diode laser was selected as the light source. ECDL lasers are semiconductor lasers that utilize external cavity tuning technology to control the output light frequency. They possess narrow linewidth, high stability, and excellent tuning performance, making them an ideal light source choice for CRDS technology. These characteristics enable ECDL lasers to provide a high-precision, reliable, and flexible light source for achieving accurate spectral analysis and measurement in CRDS technology.

[0024] The function of the fiber optic coupler is to couple the laser light emitted by the ECDL external cavity diode laser through optical fibers and then transmit it to the MZM Mach-Zeder modulator. This fiber optic coupler not only improves the stability of the system but also ensures that the laser source can be transmitted efficiently.

[0025] The laser signal is then input to an MZM Mach-Zeder modulator, where its phase and polarization are modulated under the integrated laser frequency stabilizer. The integrated laser frequency stabilizer's laser lock box integrates a waveform generator, mixer, low-pass filter, and a dual-cascaded PID controller for PDH locking, integrating the complex electronic equipment required in traditional PDH technology, greatly improving the ease and speed of system setup. The MZM Mach-Zeder modulator was specifically chosen to replace the electro-optic and acousto-optic modulators used in traditional PDH technology because it offers high-speed performance, wide bandwidth, and low power consumption, enabling very high modulation speeds and adapting to the requirements of various optical communication systems and different transmission rates. Furthermore, due to its phase modulation, the MZM Mach-Zeder modulator introduces lower distortion, maintaining high-quality optical signals and low nonlinear distortion. It also features high stability and ease of integration, with minimal impact from environmental factors, and can be easily integrated with other optical components. In summary, using the MZM Mach-Zeder modulator provides high-quality, high-efficiency optical signal transmission, bringing better performance and reliability to PDH technology.

[0026] Next is the fiber optic splitter, whose function is to split the modulated laser in two, forming a PDH beam and a probe beam, thus creating a dual-optical-path system. Figure 1 In the diagram, the two beams of light are marked with dashed and solid lines. One beam represents the PDH light, which is marked with a solid line. This light first passes through the first half-wave plate, which rotates its polarization by 180°, making it perpendicular to the other beam. Then, through the cooperation of the second half-wave plate, the Faraday rotator, and the first and second polarization beam splitters, the PDH light is ensured to be input into the ring-down cavity with a polarization state perpendicular to the other beam, so that the two beams do not interfere with each other in actual ring-down event detection.

[0027] After being injected into the ring-down cavity, the PDH light beam is reflected to form an optical signal. This signal passes through a second polarization beam splitter, a Faraday rotator, a second half-wave plate, and a first polarization beam splitter before being reflected by a mirror into a first photodetector. The photodetector receives the signal and then processes and converts it through an integrated laser frequency stabilizer. Finally, the generated electrical signal drives the ECDL external cavity diode laser, modulating the phase and frequency of the emitted laser to achieve a stable output frequency. This integrated laser frequency stabilizer modulates the ECDL external cavity diode laser at high speed.

[0028] The other beam in the dual-path is the probe beam, marked with a dashed line in the diagram. This beam is transmitted through the second polarization beam splitter into the ring-down cavity. The probe beam undergoes numerous reflections within the ring-down cavity to trigger a ring-down event. When a ring-down event occurs, a fast switch immediately cuts off the incident probe beam, and a second photodetector is used to detect the attenuation of the probe beam within the cavity in real time to obtain the data required for CRDS.

[0029] In this system, before a ring-down event occurs, the probe light and the PDH light are simultaneously incident into the cavity. When a ring-down event occurs, the system only cuts off the incident probe light, while the PDH light remains incident. This is to ensure that the PDH light can maintain the laser output frequency locked to the cavity mode in real time. This method can reduce the time required to relock the laser mode between two detections, which is close to the theoretical limit. At the same time, there is no need to worry about the PDH light affecting the probe light detection during the ring-down event and the measurement process. Due to the perpendicular polarization of the PDH light and the probe light, there is a 4000:1 splitting ratio between the two beams in actual testing, which means that there is perfect isolation between the PDH light and the probe light signal.

[0030] In summary, the entire system's workflow is as follows: The ECDL external cavity diode laser emits laser light of a specific frequency, which is input to the MZM Mach-Zeder modulator via an optical fiber coupler. Then, under the control of a signal generator, the phase and polarization of the laser light are modulated. An optical fiber splitter separates the modulated laser light into a PDH beam and a probe beam. The PDH beam is processed to form an electrical signal that can drive the ECDL external cavity diode laser for modulation. The probe beam, through reflection and decay of the ring-down cavity, triggers an event and is then converted into acquireable data by a second photodetector.

[0031] Although the entire system requires extremely precise design and rigorous execution to build and operate, the innovation and practicality of this patent are significant. The system's design enables rapid locking of cavity resonant modes, which undoubtedly greatly improves the convenience and accuracy of experiments.

[0032] PDH (Programmable Continuous Discharge) technology can rapidly detect changes in laser frequency by monitoring interference signals in the optical cavity in real time and adjust the laser frequency accordingly to maintain stability. Through this real-time frequency stability and adaptive adjustment capability of PDH mode-locking, the CRDS measurement system can counteract laser frequency fluctuations and offsets, thereby improving the accuracy and repeatability of measurement results. Combining the advantages of CRDS spectroscopy and PDH mode-locking technology fully utilizes the high sensitivity and high resolution of CRDS, while real-time stabilization and adjustment of the laser frequency via PDH mode-locking significantly improves the frequency stability of the CRDS measurement system. This innovative combination will help expand the application areas of CRDS technology and provide more accurate and reliable measurement methods for fields such as optical analysis, environmental monitoring, and chemical research. Therefore, combining CRDS spectroscopy with PDH mode-locking technology is of great significance for improving the accuracy and reliability of CRDS measurements.

[0033] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cavity ring-down spectral system based on dual-path PDH mode-locking technology, characterized in that, include: Laser frequency feedback control module, laser phase polarization modulation module, and beam splitting modulation module; The laser frequency feedback control module includes an external cavity diode laser, a fiber coupler, and an integrated laser frequency stabilizer; the external cavity diode laser serves as a light source, emitting a laser of a specific frequency into the fiber coupler, which is used for fiber coupling of the laser. The laser phase polarization modulation module includes a Mach-Zeder modulator. The laser, after being coupled by an optical fiber coupler, is delivered to the Mach-Zeder modulator and its phase and polarization are modulated under the integrated laser frequency stabilizer. The integrated laser frequency stabilizer's laser lock box integrates a waveform generator, mixer, low-pass filter, and dual cascaded PID controller for PDH lock-up; The beam splitting and modulation module includes an optical fiber splitter and a ring-down cavity. The modulated laser enters the optical fiber splitter, which splits the modulated laser into two beams to form a PDH beam and a probe beam. After the PDH beam is injected into the ring-down cavity, it is reflected to form an optical signal. The optical signal is processed and converted by an integrated laser frequency stabilizer, and finally drives the external cavity diode laser through the generated electrical signal. The probe light enters the ring-down cavity and is reflected countless times within the cavity to trigger a ring-down event. The beam splitting modulation module also includes a first half-wave plate, which is used to rotate the polarization modulation of the PDH light by 180° so that it is perpendicular to the probe light. The beam splitting and modulation module also includes a first polarization beam splitter, a second half-wave plate, a Faraday rotator, and a second polarization beam splitter; after the PDH light polarization is rotated by 180°, it is sequentially input into the ring-down cavity through the first polarization beam splitter, the second half-wave plate, the Faraday rotator, and the second polarization beam splitter. It also includes a first photodetector. After the PDH light is input into the ring-down cavity, it is reflected to form an optical signal. The optical signal passes through the second polarization beam splitter, Faraday rotator, second half-wave plate and first polarization beam splitter in sequence and is reflected by a mirror into the first photodetector. The optical signal is received by the first photodetector and then the first photodetector inputs the optical signal into the integrated laser frequency stabilizer. It also includes a fast switch and a second photodetector. When a ring-down event occurs, the fast switch immediately cuts off the incident probe light, and the second photodetector is used to detect the attenuation of the probe light in the cavity in real time to obtain the ring-down spectrum data of the optical cavity.

2. The cavity ring-down spectral system based on dual-path PDH mode-locking technology according to claim 1, characterized in that: The probe light is transmitted through the second polarization beam splitter into the decay cavity.

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