CN106814516A - A kind of continuous wave optical parametric oscillator of pumping resonance - Google Patents

Abstract

The invention discloses a continuous wave optical parametric oscillator with pump resonance, comprising: a single-frequency pump laser, a beam collimation unit, a first optical isolator, a pump input coupling mirror, a first concave mirror, a nonlinear crystal, A second concave mirror, a dichroic mirror, an optical bandpass filter and an OPO signal light output coupling mirror. In the present invention, the nonlinear crystal is directly placed between two concave mirrors, and a smaller spot area of the signal and pump light can be obtained by selecting a concave mirror with a short focal length; the signal optical cavity and the pump optical cavity are increased by using the folded cavity The cavity length can further reduce the spot area of the signal and pump light in the nonlinear crystal, and achieve an extremely low oscillation threshold, which can be lower than 60 mW or even lower than 30 mW. The low-threshold continuous wave optical parametric oscillator can reduce the power requirements of the pump laser, and has a relatively simple structure, which is easy to realize miniaturization and practicality.

Description

A Pump Resonant Continuous Wave Optical Parametric Oscillator

technical field

The invention relates to the field of optical parametric oscillators, more specifically, to a continuous wave optical parametric oscillator with pump resonance.

Background technique

The optical parametric oscillator (OPO) uses the second-order nonlinear effect to convert the incident laser light, called the pump light, into two outgoing laser lights with different frequencies, one of which has a higher frequency is called the signal light, and the other is called the signal light. A lower frequency is called the idler light, and the frequency of the pump light is equal to the sum of the frequencies of the signal light and the idler light; the optical parametric oscillator is used to expand the output band of the laser radiation and generate broadband tunable laser radiation.

A continuous wave optical parametric oscillator generally uses a fiber amplifier as a pump source; a longer periodically polarized nonlinear crystal, such as a periodically poled lithium niobate crystal (Periodically poled lithium niobate, PPLN), is used as a gain medium to reduce Oscillation threshold, at this time the oscillation threshold is generally 2-3W. The configuration and experimental results of a typical continuous wave single-resonant optical parametric oscillator are shown in the literature E.Andrieux, T.Zanon, M.Cadoret, A.Rihan, and J.-J.Zondy, "500GHz mode-hop-free idler tuning range with a frequency-stabilized singly resonant optical parametric oscillator," Opt. Lett. 36, 1212–1214 (2011).

The pump resonance technology is used to make the pump laser oscillate in the resonator, which can obtain higher pump power density in the optical parametric oscillator and greatly reduce the oscillation threshold of the continuous wave optical parametric oscillator. The oscillation threshold of the continuous wave optical parametric oscillator with pump resonance obtained in the current experiment is about 400 milliwatts. The configuration and experimental results of a typical pump resonant continuous wave optical parametric oscillator are shown in the literature F.Muller, A.Popp, F.Kuhnemann, and S.Schiller, "Transportable, highly-sensitive photoacoustic spectrometer based on continuous-wave dual- cavity optical parametric oscillator,” Opt. Express 11, 2820–25 (2003).

At present, the oscillation threshold of the continuous wave optical parametric oscillator using the pump resonance technology is still high, mainly because the pump light of the optical parametric oscillator has a large spot area in the existing configuration; it is necessary to use a watt-level output power A single-frequency laser and a power amplifier system are used as the pump source, so the pump system is complex and bulky.

Contents of the invention

The purpose of the present invention is to provide a continuous wave optical parametric oscillator with a lower oscillation threshold (which can be lower than 60 milliwatts or even 30 milliwatts), so as to overcome the problems of high oscillation threshold and high requirements on the output power of the pumping system in the prior art .

The invention provides a continuous wave optical parametric oscillator with pump resonance, comprising: a single-frequency pump laser, a beam collimation unit, a first optical isolator, a pump input coupling mirror, a first concave mirror, a nonlinear crystal, The second concave mirror, dichroic mirror, optical bandpass filter and OPO signal light output coupling mirror; the single-frequency pump laser is used as the pump source to emit OPO pump light, and the OPO pump light is collimated by the beam collimation unit and then After passing through the optical isolator, the collimated OPO pump light sequentially passes through the pump input coupling mirror, the first concave mirror, the nonlinear crystal, the second concave mirror and the dichroic mirror, and after the pumping input coupling mirror, Oscillation is formed in the first resonant cavity composed of the first concave mirror, nonlinear crystal, second concave mirror and dichroic mirror; the pump light oscillating in the first resonant cavity passes through the nonlinear crystal to generate OPO signal light and OPO idler light ; The OPO signal light is formed in the second resonant cavity formed by the pump input coupling mirror, the first concave mirror, the nonlinear crystal, the second concave mirror, the dichroic mirror, the optical bandpass filter and the OPO signal light output coupling mirror Oscillation; the pump light oscillating in the second resonator passes through the nonlinear crystal to generate OPO signal light and OPO idler light; the OPO signal light is output through the OPO signal light output coupling mirror, and the OPO idler light passes through the first concave mirror and the second The two concave mirrors are output respectively.

Furthermore, the pump input coupling mirror is coated with OPO pump light partial reflection, signal light high reflection film; the first concave mirror and the second concave mirror are coated with OPO pump light and signal light high reflection, idler light enhancement film Transparent film; the dichroic mirror is coated with OPO pump light high reflection and signal light anti-reflection coating; the OPO signal light output coupling mirror is coated with OPO signal light partial reflection film; the nonlinear crystal is coated with OPO pump, signal and idler Frequency light anti-reflection coating; Among them, the high-reflection coating refers to a film with a power reflectance greater than 99%, the anti-reflection coating refers to a film with a power transmittance greater than 97%, and the partially reflective film refers to a film with a reflectance of less than 15%.

Furthermore, the beam collimation unit is used to realize the collimation of the single-frequency pump laser, and realize the matching of the pump laser and the spatial mode of the OPO pump optical resonator by changing the radius of the pump spot.

Furthermore, the nonlinear crystal is periodically poled lithium niobate crystal, potassium titanyl phosphate crystal or other crystals capable of generating nonlinear gain.

Furthermore, the continuous wave optical parametric oscillator further includes a pump cavity length feedback control module, which is used to maintain the resonance state between the single-frequency pump laser and the OPO pump optical resonator.

Furthermore, the pump cavity length feedback control module includes: a low-pass filter, a mixer, a Pockels chamber, a second optical isolator, a servo amplifier, an oscillator, a phase shifter, and a photodetector; a collimator After the pump light passes through the Faraday isolator, the Pockels chamber and the second optical isolator in turn, it oscillates in the first resonant cavity, and the reflected part of the light is output through the pump input coupling mirror, and then passes through the The second optical isolator is received by the photon detector and generates an electrical signal, which is transmitted to the mixer; part of the disturbance signal generated by the oscillator acts on the Pockels chamber, and the other part is transmitted after passing through the phase shifter The electrical signal to the mixer and from the photodetector is mixed, and the mixed signal is transmitted to the low-pass filter, and the low-frequency signal passed through the low-pass filter is transmitted to the servo amplifier, and the amplified signal after the servo amplifier is used for controlling and regulating the single-frequency pump laser.

Furthermore, the second optical isolator includes: a polarization beam splitter and a quarter wave plate arranged in sequence, the polarization beam splitter turns the input light into a linearly polarized light with a specific polarization direction, and has a beam splitting effect on the light; One of the wave plates is used to convert the incident linearly polarized light whose polarization direction is at an angle of 45 degrees to the quarter wave plate into circularly polarized light, and can also convert the input circularly polarized light into a polarization direction at 45 degrees to the quarter wave plate angled linear polarizers.

Furthermore, the nonlinear crystal is placed in the OPO pumping optical resonant cavity, and the pumping light oscillating in the cavity passes through the nonlinear crystal to generate the gain of the OPO signal light and the idler light; the OPO signal light is generated by the pump input coupling mirror, the first concave mirror, the nonlinear crystal, the second concave mirror, the dichroic mirror, the optical bandpass filter and the OPO signal light output coupling mirror The signal light resonant cavity of the OPO forms an oscillation, the OPO signal light generated is output through the OPO signal light output coupling mirror, and the OPO idle frequency light generated is output through the first concave mirror and the second concave mirror; The purpose of the first concave mirror and the second concave mirror is to realize the focusing of the OPO signal and the pump light in the nonlinear crystal, so as to reduce the oscillation threshold of the OPO.

Furthermore, the single-frequency pump laser can be a single-frequency semiconductor, solid-state or fiber laser, or a single-frequency laser and amplifier system. An optical isolator is used to inhibit the pump laser light from being reflected back to the single frequency pump laser. The pump input coupling mirror is used to realize the coupling of the single-frequency pump laser to the OPO pump optical resonator. The reflectivity of the pump input coupling mirror to the OPO pump light is set to the sum of the one-way losses of the OPO pump light by other components except the pump input coupling mirror in the OPO pump optical resonator, Optimum pump light incoupling efficiency can be achieved. The optical bandpass filter is used to realize bandpass filtering of the OPO signal light, so as to realize single-frequency selection and wavelength tuning of the OPO signal light, and can be a Fabry-Perot cavity, a birefringent filter or other optical filter elements.

Compared with the existing continuous wave optical parametric oscillator using pump resonance technology, the present invention places the nonlinear crystal directly between two concave mirrors, and can obtain a smaller OPO by selecting a concave mirror with a short focal length. The spot area of the signal and pump light; using the folded cavity to increase the cavity length of the OPO signal cavity and the pump cavity can further reduce the spot area of the OPO signal and pump light in the nonlinear crystal, and achieve extremely low OPO oscillation Threshold, its oscillation threshold can be lower than 60 mW or even lower than 30 mW. The low-threshold continuous wave optical parametric oscillator can reduce the power requirements of the pump laser, and has a relatively simple structure, which is easy to realize miniaturization and practicality.

Description of drawings

FIG. 1 is a schematic structural diagram of a continuous wave optical parametric oscillator with pump resonance provided by an embodiment of the present invention.

Fig. 2 is a schematic structural diagram of a pump cavity length feedback control module in a continuous wave optical parametric oscillator with pump resonance provided by an embodiment of the present invention.

Among them, 1 is the single-frequency pump laser, 2 is the beam collimation unit, 3 is the first optical isolator, 4 is the pump input coupling mirror, 5 is the first concave mirror, 6 is the nonlinear crystal, 7 is the second Concave mirror, 8 is a dichroic mirror, 9 is an optical bandpass filter, 10 is an OPO signal light output coupling mirror, 11 is a low-pass filter, 12 is a mixer, 13 is a Pockels chamber, 14 is a second An optical isolator, 15 is a servo amplifier, 16 is an oscillator, 17 is a phase shifter, and 18 is a photodetector.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not constitute a conflict with each other.

In the present invention, the nonlinear crystal is directly placed between two concave mirrors, and a smaller spot area of the OPO signal and pump light can be obtained by selecting a concave mirror with a short focal length; The cavity length of the optical cavity can further reduce the spot area of the OPO signal and pump light in the nonlinear crystal, and achieve an extremely low OPO oscillation threshold, which can be lower than 60 mW or even lower than 30 mW. The low-threshold continuous wave optical parametric oscillator can reduce the power requirements of the pump laser, and has a relatively simple structure, which is easy to realize miniaturization and practicality.

The pump resonant continuous wave optical parametric oscillator includes: a single-frequency pump laser 1, a beam collimator unit 2, an optical isolator 3, a pump input coupling mirror 4, a first concave mirror 5, a nonlinear crystal 6, a second Concave mirror 7, dichroic mirror 8, optical bandpass filter 9, OPO signal light output coupling mirror 10 and pump cavity length feedback control module, wherein,

The single-frequency pumping laser 1 is used as the pumping source of the optical parametric oscillator. After passing through the beam collimation unit 2 and the optical isolator 3, the OPO pumping light sequentially passes through the pump input coupling mirror 4, the first concave mirror 5, A nonlinear crystal 6, a second concave mirror 7 and a dichroic mirror 8, and in a resonant cavity composed of a pump input coupling mirror 4, a first concave mirror 5, a nonlinear crystal 6, a second concave mirror 7 and a dichroic mirror 8 form oscillations. The pump light oscillating in the cavity passes through the nonlinear crystal 6, which will generate the gain of OPO signal light and idler light; the OPO signal light passes through the pump input coupling mirror 4, the first concave mirror 5, the nonlinear crystal 6, the second Two concave mirrors 7, dichroic mirror 8, optical bandpass filter 9 and OPO signal light output coupling mirror 10, and by pumping input coupling mirror 4, first concave mirror 5, nonlinear crystal 6, second concave mirror 7 , dichroic mirror 8 , optical bandpass filter 9 and OPO signal light output coupling mirror 10 to form an oscillation in the signal light resonant cavity of the OPO. The function of the resonant cavity is to allow the light in the cavity to obtain positive feedback, and to select light with a wavelength that can resonate in the resonant cavity. The generated OPO signal light is output through the OPO signal light output coupling mirror 10, and the OPO idler light generated is output through the first concave mirror 5 and the second concave mirror 7; the purpose of using the first concave mirror 5 and the second concave mirror 7 is to Realize the focusing of the OPO signal and the pump light in the nonlinear crystal 6, the signal light and the pump light are focused in the crystal, so that the spot area of the two kinds of light in the crystal is small, if the two kinds of light cannot be focused in the nonlinear In the crystal 6, since the spot area at the focus position is the smallest, the spot area of the signal light and the pump light in the nonlinear crystal 6 will be smaller than the spot area when these two kinds of light are focused in the nonlinear crystal 6 at the same time The area should be large. According to the literature Robert W. Boyd, Nonlinear Optics 3rd ed. We know that the smaller the spot area of the resonant beam in the nonlinear crystal 6, the smaller the oscillation threshold. Therefore, focusing the OPO signal light and pump light in the nonlinear crystal 6 can reduce the oscillation threshold of the OPO.

The pump input coupling mirror 4 is used as the input coupling mirror of the pump light and also as an end reflector of the pump light resonator, so that the pump light is reflected back and forth inside the resonator, and the first concave mirror 5 reflects the pump light And make the pump light converge, the function of the nonlinear crystal 6 is to use the second-order nonlinear effect to generate idler light, the second concave mirror 7 reflects the pump light and makes the pump light converge, and the dichroic mirror 8 reflects the pump light at the same time Make the signal light pass through; change the cavity length of the OPO pumping optical resonator by adjusting the position of the dichroic mirror 8, or adjust the wavelength of the single-frequency pumping laser 1, so that the pumping light oscillates in the OPO pumping optical resonant cavity, The enhanced pump light power density is realized in the OPO resonator; the resonance state of the single-frequency pump laser 1 and the OPO pump light resonator is maintained through the pump cavity length feedback control module.

In the embodiment of the present invention, the pump input coupling mirror 4 is coated with a film with an OPO pump light reflectance of 96% and a signal light reflectance greater than 99.5%; the first concave mirror 5 and the second concave mirror 7 are coated with the same film, Coated with a film with OPO pump light and signal light reflectivity greater than 99.5% and idler light reflectivity less than 20%; dichroic mirror 8 coated with OPO pump light reflectivity greater than 99.5% and signal light reflectivity less than 0.5% film; the OPO signal light output coupling mirror 10 is coated with a film with a reflectivity of 95% for the OPO signal light; both surfaces of the nonlinear crystal 6 are coated with the reflectivity of these three kinds of light such as OPO pump light, signal light and idler light Both are less than 2% of the film.

A concave mirror with a short focal length is selected. In this embodiment, a concave mirror with a radius of curvature of 75 mm is selected. The general principle for choosing the radius of curvature of the concave mirror is to make the oscillation threshold low.

As an embodiment of the present invention, nonlinear crystal 6 can select PPLN, when the wavelength of the pumping light (ie laser) of OPO is 1040 nanometers, when the polarization period of PPLN is 30.2 microns, the wavelength of signal light and idler light 1517 nm and 3305 nm, respectively. Locking the wavelength of the OPO pump light, changing the wavelength of the signal light of the OPO by tuning the optical bandpass filter 9 and synchronously changing the polarization cycle or temperature of the PPLN can realize the tuning of the wavelength of the idler light output by the OPO.

In the embodiment of the present invention, the single-frequency pump laser 1 may be a single-frequency semiconductor, solid-state or fiber laser, or a single-frequency laser and amplifier system. The beam collimation unit 2 is used to realize the collimation of the single-frequency pump laser 1, and realize the matching of the pump laser and the spatial mode of the OPO pump optical resonator by changing the radius of the pump spot. The optical isolator 3 is used to prevent the pump laser from being reflected back to the single-frequency pump laser. The pump input coupling mirror 4 is used to realize the coupling of the single-frequency pump laser 1 to the OPO pump optical resonator. The reflectivity of the pump-in coupling mirror 4 to the OPO pump light is set to be the sum of the one-way losses of the OPO pump light to other elements in the OPO pump optical resonator except the pump-in coupling mirror 4, which can realize Optimum pump light incoupling efficiency. The nonlinear crystal 6 can be periodically poled lithium niobate crystal, potassium titanyl phosphate crystal or other crystals capable of generating nonlinear gain. The optical band-pass filter 9 is used to realize the band-pass filtering of the OPO signal light, so as to realize single-frequency selection and wavelength tuning of the OPO signal light, and can be a Fabry-Perot cavity, a birefringent filter or other optical filters element.

In the embodiment of the present invention, the pump cavity length feedback control module is used to realize the resonance state between the single-frequency pump laser 1 and the OPO pump optical resonator, wherein the pump light generated by the single-frequency pump laser 1 passes through The collimator 2 obtains collimated pump light, and the collimated pump light passes through the Faraday isolator 3, the Pockels chamber 13, the second optical isolator 14, and the second optical isolator 14 is composed of a polarization beam splitter and a quarter-wave plate, the light from the isolator 14 passes through the pump input coupling mirror 4, and the pump light after passing through the pump input coupling mirror 4 passes through the pump input coupling mirror 4 The structure omitted later is reflected back, and part of the light is output through the pump input coupling mirror 4 , and after passing through the second optical isolator 14 , it is received by the photon detector 18 to generate an electrical signal, which is transmitted to the mixer 12 . Oscillator 16 generates a disturbance signal, part of which acts on the Pockels chamber, part of which is transmitted to mixer 12 after passing through phase shifter 17 and mixed with the electrical signal from photodetector 18, and the mixed signal Transmitted to the low-pass filter 11, the low-frequency signal passed through the low-pass filter 11 is transmitted to the servo amplifier 15, and the amplified signal after passing through the servo amplifier 15 will directly act on the single-frequency pump laser 1 to control and adjust the single-frequency pump The role of Pu laser 1.

When the pump cavity length feedback control module described in Fig. 2 is applied to the OPO described in Fig. 1, the components in Fig. 1 and Fig. 2 are combined in combination with the connection relationship between the optical paths and devices in Fig. Together. Specifically, add a Pockels chamber 13 and a second optical isolator 14 between the Faraday isolator 3 described in FIG. 1 and the pump input coupling mirror 4, a low-pass filter 11, a mixer 12, a servo Amplifier 15 , oscillator 16 , phase shifter 17 and photodetector 18 can be added to FIG. 1 according to the connection relationship described in FIG. 2 .

The pump cavity length feedback control module takes the resonant frequency of the resonant cavity as the reference frequency standard, and uses phase modulation technology to generate a sideband on both sides of the laser frequency. The optical signal reflected by the reference resonant cavity is compared with the modulated signal and filtered After amplification, the error signal that the laser frequency deviates from the resonance frequency of the reference cavity is obtained, and the error signal is used to drive the feedback control system to adjust a certain parameter of the laser to stabilize the laser frequency at the resonance frequency of the optical cavity.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. within range.

Claims (7)

1. A continuous wave optical parametric oscillator for pump resonance, characterized in that it comprises: a single-frequency pump laser (1), beam collimation unit (2), the first optical isolator (3), pump input coupling Mirror (4), first concave mirror (5), nonlinear crystal (6), second concave mirror (7), dichroic mirror (8), optical bandpass filter (9) and OPO signal light output coupling mirror ( 10); A single-frequency pumping laser (1) is used as a pumping source to emit OPO pumping light, and the OPO pumping light is collimated by a beam collimating unit (2) and then passes through the optical isolator (3), the collimated The OPO pump light passes through the pump input coupling mirror (4), the first concave mirror (5), the nonlinear crystal (6), the second concave mirror (7) and the dichroic mirror (8) in sequence, and is input by the pump Oscillation is formed in the first resonant cavity formed by the coupling mirror (4), the first concave mirror (5), the nonlinear crystal (6), the second concave mirror (7) and the dichroic mirror (8); The pumping light oscillating in the first resonator passes through the nonlinear crystal (6) to generate OPO signal light and OPO idler light; the OPO signal light is pumped into the coupling mirror (4), the first concave mirror (5), Oscillation is formed in the second resonant cavity formed by the nonlinear crystal (6), the second concave mirror (7), the dichroic mirror (8), the optical bandpass filter (9) and the OPO signal light output coupling mirror (10); The pumping light oscillating in the second resonator passes through the nonlinear crystal (6) to generate OPO signal light and OPO idler light; the OPO signal light is output through the OPO signal light output coupling mirror (10), and the OPO idler light passes through the first The concave mirror (5) and the second concave mirror (7) output respectively. 2. continuous wave optical parametric oscillator as claimed in claim 1, is characterized in that, described pump input coupling mirror (4) is coated with OPO pumping light partial reflection, signal light high reflection film; Described first concave mirror ( 5) and the second concave mirror (7) are coated with OPO pump light and signal light high reflection, idler light anti-reflection film; the dichroic mirror (8) is coated with OPO pump light high reflection, signal light anti-reflection film; The OPO signal light output coupling mirror (10) is plated with an OPO signal light partial reflection film; the nonlinear crystal (6) is plated with an OPO pumping, signal and idler light anti-reflection film; Among them, the high-reflection film refers to a film with a power reflectance greater than 99%, the anti-reflection film refers to a film with a power transmittance greater than 97%, and the partially reflective film refers to a film with a reflectance of less than 15%. 3. The continuous wave optical parametric oscillator according to claim 1, characterized in that, the beam collimation unit (2) is used to realize the collimation of the single-frequency pumping laser, and by changing the pump spot The radius is used to achieve the matching of the pump laser and the spatial mode of the OPO pump optical resonator. 4. The continuous wave optical parametric oscillator as claimed in claim 1, characterized in that, the nonlinear crystal (6) is periodically poled lithium niobate crystal, potassium titanyl phosphate crystal or other crystals that can produce nonlinear gain crystals. 5. The continuous wave optical parametric oscillator according to claim 1, wherein the continuous wave optical parametric oscillator also includes a pump cavity length feedback control module for maintaining the single-frequency pumping laser and OPO pumping The resonance state of the optical cavity. 6. continuous wave optical parametric oscillator as claimed in claim 5, is characterized in that, described pump cavity length feedback control module comprises: low-pass filter (11), frequency mixer (12), Pockels chamber (13), the second optical isolator (14), servo amplifier (15), oscillator (16), phase shifter (17) and photodetector (18); After the collimated pump light passes through the Faraday isolator (3), the Pockels chamber (13) and the second optical isolator (14) in turn, it oscillates in the first resonant cavity, and the reflected part The light is output through the pump input coupling mirror (4), and then received by the photon detector (18) after passing through the second optical isolator (14) to generate an electrical signal, which is transmitted to the mixer (12); Part of the disturbance signal generated by the oscillator (16) acts on the Pockels chamber (13), and the other part is transmitted to the mixer (12) and from the photodetector (18) after being passed through the phase shifter (17). The electric signal is mixed, and the mixed signal is transmitted to the low-pass filter (11), and the low-frequency signal passed through the low-pass filter (11) is transmitted to the servo amplifier (15), and the amplification after the servo amplifier (15) The signal is used to control and regulate the single frequency pump laser (1). 7. continuous wave optical parametric oscillator as claimed in claim 6, is characterized in that, described second optical isolator (14) comprises: polarized beam splitter and quarter wave plate that are arranged in sequence, polarized beam splitter will input The light becomes linearly polarized light with a specific polarization direction and has a beam splitting effect on the light; the quarter-wave plate is used to convert the incident linearly polarized light whose polarization direction is at an angle of 45 degrees to the quarter-wave plate into circularly polarized light, also The input circularly polarized light can be converted into linearly polarized light whose polarization direction is at an angle of 45 degrees to the quarter-wave plate.