Device and method for generating multi-frequency coherent laser

Abstract

A device and method for generating multi-frequency coherent laser light, comprising: a laser for generating seed light; a signal generator for generating a modulation signal and a demodulation signal; a servo module for generating a control signal; an intensity-modulated electro-optical modulator for receiving the modulation signal and the control signal and modulating the seed light to generate a first modulated optical signal; a polarization-maintaining optical fiber beam splitter for splitting the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio; a preprocessing module for receiving the demodulation signal and the feedback optical signal and generating an error signal through frequency mixing and phase shifting; an optical fiber amplifier for power-amplifying the second modulated optical signal; and a periodically polarized crystal for frequency-doubling the power-amplified second modulated optical signal to obtain multi-frequency coherent laser light, wherein the frequency difference between two adjacent frequency components in the multi-frequency coherent laser light is twice the frequency of the radio frequency signal.

Description

Technical Field

[0001] The present disclosure relates to the fields of optoelectronic technology and quantum information, and in particular to a device and method for generating multi-frequency coherent lasers. Background Art

[0002] Manipulating the quantum state of a physical system with the help of stimulated Raman transitions is one of the basic methods for achieving quantum control. The key technology for achieving stimulated Raman transitions is to obtain multi-frequency coherent lasers with a specific frequency difference. At present, there are three main technical means to obtain multi-frequency coherent lasers with a specific frequency difference: First, using a phase-type electro-optical modulator (Electro-Optic Modulators, EOM) to directly modulate the laser to obtain a multi-frequency coherent laser with a specific frequency difference, and using a multi-frequency coherent laser with a specific frequency difference to achieve stimulated Raman transitions. Second, using the master-slave laser locking method, the high-frequency EOM modulates the sideband of the laser generated by the master laser, and through the sideband injection locking technology, the slave laser is used to lock the sideband of the laser generated by the master laser, which can effectively eliminate the carrier component and obtain a multi-frequency coherent laser with a specific frequency difference. Third, by using a pulsed mode-locked laser, the laser spectrum output by the mode-locked laser contains a series of frequency comb teeth with an interval equal to its repetition frequency. By frequency shifting with an additional acousto-optical modulator (AOM), two frequency comb teeth with a frequency interval equal to the corresponding energy level splitting frequency can always be generated in two Raman beams, thereby obtaining a multi-frequency coherent laser with a specific frequency difference.

[0003] The above technical means have the following problems: the obtained multi-frequency coherent laser frequency difference tuning range is limited, and the used equipment is difficult to maintain. Summary of the Invention

[0004] In view of this, the main purpose of the present disclosure is to provide a device and method for generating multi-frequency coherent laser, in order to partially solve at least one of the above-mentioned technical problems.

[0005] In order to achieve the above objectives, one aspect of the present disclosure provides a device for generating multi-frequency coherent laser light, comprising:

[0006] A laser is used to generate seed light, wherein the seed light is linearly polarized light with adjustable wavelength; a signal generator is used to generate a modulation signal and a demodulation signal with the same phase and the same frequency, wherein the signal frequency of the modulation signal and the demodulation signal is adjustable; a servo module is used to generate a control signal, wherein the amplitude of the control signal is adjustable; an intensity modulation electro-optical modulator is used to receive the modulation signal and the control signal, and modulate the seed light to generate a first modulated optical signal, wherein the first modulated optical signal includes a modulated sideband optical signal; a polarization-maintaining optical fiber beam splitter is used to divide the first modulated optical signal into a first modulated optical signal according to a preset power ratio. The optical signal is divided into a second modulated optical signal and a feedback optical signal, wherein the second modulated optical signal includes a modulated sideband optical signal; a preprocessing module is used to receive the demodulated signal and the feedback optical signal, and generate an error signal through frequency mixing and phase shifting, so that the servo module receives the error signal and generates the control signal based on the error signal; an optical fiber amplifier is used to amplify the power of the second modulated optical signal; and a periodically poled crystal is used to perform frequency doubling processing on the power-amplified second modulated optical signal to obtain multi-frequency coherent laser light, wherein the frequency difference between two adjacent lasers in the multi-frequency coherent laser light is twice the frequency of the radio frequency signal.

[0007] According to an embodiment of the present disclosure, the intensity-modulated electro-optical modulator uses a Mach-Zehnder interferometer structure, which includes two working arms of equal length, a radio frequency signal modulation port, and a control signal modulation port; the radio frequency signal modulation port is used to receive the modulation signal, and the control signal modulation port receives the control signal; the two working arms of equal length change the optical path difference of the seed light transmitted on the two working arms of equal length based on the modulation signal and the control signal to generate the first modulated optical signal.

[0008] According to an embodiment of the present disclosure, the frequency of the modulation signal and the demodulation signal generated by the signal generator is half the energy level splitting frequency of the corresponding particles undergoing stimulated Raman transition using the multi-frequency coherent laser.

[0009] According to an embodiment of the present disclosure, the voltage of the control signal generated by the servo module is a bias half-wave voltage that enables the Mach-Zehnder interferometer structure to operate at a minimum operating point.

[0010] According to an embodiment of the present disclosure, the preprocessing module includes:

[0011] A photodetector is used to receive the above-mentioned feedback optical signal and generate a feedback electrical signal; a microwave phase shifter is used to shift the phase of the above-mentioned feedback electrical signal to generate a phase-shifted feedback electrical signal; a mixer is used to mix the above-mentioned feedback electrical signal and the above-mentioned demodulated signal to generate a mixed signal; a low-pass filter is used to filter the above-mentioned mixed signal to generate an error signal, and the above-mentioned error signal is used to characterize the optical path difference of the seed light transmitted on the two equal-length working arms of the above-mentioned Mach-Zehnder interferometer structure.

[0012] According to an embodiment of the present disclosure, the apparatus for generating multi-frequency coherent laser light further includes:

[0013] A focusing lens is provided between the optical fiber amplifier and the periodically poled crystal. The focusing lens is used to control the second modulated optical signal to enter the periodically poled crystal in its entirety. A collimating lens is provided at the other end of the periodically poled crystal relative to the optical fiber amplifier. The collimating lens is used to collimate the multi-frequency coherent laser.

[0014] According to the embodiment of the present disclosure, polarization-maintaining optical fiber is used to connect the above-mentioned laser and the above-mentioned intensity-modulated electro-optical modulator, between the above-mentioned intensity-modulated electro-optical modulator and the above-mentioned polarization-maintaining optical fiber splitter, between the above-mentioned polarization-maintaining optical fiber splitter and the above-mentioned optical fiber amplifier, and between the above-mentioned polarization-maintaining optical fiber splitter and the above-mentioned preprocessing module.

[0015] According to an embodiment of the present disclosure, coaxial cables are used to connect the signal generator and the intensity modulation electro-optical modulator, the signal generator and the pre-processing module, the pre-processing module and the servo module, and the servo module and the intensity modulation electro-optical modulator.

[0016] Another aspect of the present disclosure provides a method for generating multi-frequency coherent laser light, comprising:

[0017] A laser is used to generate seed light, which is linearly polarized light with adjustable wavelength; a signal generator is used to generate a modulation signal and a demodulation signal with the same phase and the same frequency, and the signal frequency of the modulation signal and the demodulation signal is adjustable; the frequency of the modulation signal is equal to the frequency of the demodulation signal; a servo module is used to generate a control signal, and the amplitude of the control signal is adjustable; an intensity modulation electro-optical modulator is used to receive the modulation signal and the control signal, and modulate the seed light to generate a first modulated optical signal, and the first modulated optical signal includes a modulated sideband optical signal; a polarization-maintaining optical fiber beam splitter is used to split the above-mentioned optical signals according to a preset power ratio. The first modulated optical signal is divided into a second modulated optical signal and a feedback optical signal, wherein the second modulated optical signal includes a modulated sideband optical signal; a preprocessing module is used to receive the demodulated signal and the feedback optical signal, and an error signal is generated through frequency mixing and phase modulation, so that the servo module receives the error signal and generates the control signal based on the error signal; an optical fiber amplifier is used to amplify the power of the second modulated optical signal; and a periodically poled crystal is used to perform frequency doubling processing on the power-amplified second modulated optical signal to obtain a multi-frequency coherent laser, wherein the frequency difference between two adjacent lasers in the multi-frequency coherent laser is twice the frequency of the radio frequency signal.

[0018] According to an embodiment of the present disclosure, the servo module receives the error signal and generates the control signal based on the error signal, including:

[0019] The servo module generates the control signal through a PID algorithm based on the error signal generated by the preprocessing module. The voltage of the control signal is a bias half-wave voltage that enables the Mach-Zehnder interferometer structure to operate at the minimum operating point.

[0020] Based on the above technical solutions, it can be seen that the embodiments of the present disclosure have the following beneficial effects compared to the prior art:

[0021] The device for generating multi-frequency coherent laser provided by the present disclosure uses optical fibers and optical fiber devices, does not use reflectors, and has the same stabilization effect as optical fiber lasers. Therefore, the power and direction of the generated multi-frequency coherent laser are very stable, and the multi-frequency coherent laser has the characteristics of high stability.

[0022] The device for generating multi-frequency coherent laser provided by the present disclosure can simultaneously generate multi-frequency coherent laser in the visible band and the near-infrared band.

[0023] The device for generating multi-frequency coherent laser provided by the present disclosure uses an intensity-modulated electro-optical modulator with a wide working bandwidth and a periodically polarized crystal to flexibly adjust the frequency difference of generating multi-frequency coherent laser.

[0024] The device for generating multi-frequency coherent laser provided by the present disclosure uses an intensity-modulated electro-optical modulator, which can achieve the effect of expanding the frequency difference range of the multi-frequency coherent light and eliminating redundant frequency components through modulation.

[0025] The device for generating multi-frequency coherent laser provided by the present disclosure only requires one laser to generate seed light, and has a simple structure, low cost, and easy maintenance.

[0026] The device for generating multi-frequency coherent laser provided by the present disclosure is suitable for ion-atom mixing systems or heterogeneous ion mixing systems, that is, it can be used to simultaneously perform coherent manipulation of the quantum states of ions and atoms or heterogeneous ions, which is impossible to achieve with existing technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematically shows a block diagram of an apparatus for generating multi-frequency coherent laser light according to an embodiment of the present disclosure;

[0028] Figure 2 Schematically shows a block diagram of an apparatus for generating multi-frequency coherent laser light according to another embodiment of the present disclosure;

[0029] Figure 3 A schematic diagram showing frequency components contained in a first modulated optical signal output by an intensity modulation electro-optical modulator according to an embodiment of the present disclosure is shown;

[0030] Figure 4 The figure schematically shows the frequency components contained in the multi-frequency coherent laser output by the periodically poled crystal according to the embodiment of the present disclosure.

[0031] 1-Laser;

[0032] 2-Polarization-maintaining fiber;

[0033] 3-First fiber coupling head and connected polarization-maintaining fiber;

[0034] 4-intensity modulation electro-optical modulator, 401-modulation signal modulation port, 402-control signal modulation port;

[0035] 5-polarization-maintaining fiber splitter, 501-input end fiber, 502-second modulated optical signal output end fiber, 503-feedback optical signal output end fiber;

[0036] 6-Fiber amplifier;

[0037] 7-focusing lens;

[0038] 8-Periodically poled crystal;

[0039] 9-collimating lens;

[0040] 10- optical beam splitter;

[0041] 11- Second fiber coupling head and connected polarization-maintaining fiber;

[0042] 12- low-pass filter;

[0043] 13-servo module, 131-control signal, 132-error signal;

[0044] 14- Photodetector;

[0045] 15- third optical fiber coupling head and connected polarization-maintaining optical fiber;

[0046] 16-Signal generator, 161-Modulation signal, 162-Demodulation signal, 163-Power beam splitter;

[0047] 17- mixer;

[0048] 18-Microwave phase shifter;

[0049] 19-Preprocessing module. DETAILED DESCRIPTION

[0050] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0051] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprise," "include," etc. used herein indicate the presence of the features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0052] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0053] When expressions such as "at least one of A, B and C, etc." are used, they should generally be interpreted in accordance with the meaning of the expression commonly understood by those skilled in the art (for example, "a system having at least one of A, B and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0054] The methods for obtaining multi-frequency coherent lasers with specific frequency differences can be divided into direct modulation methods using a phase-type electro-optical modulator, master-slave laser locking methods, and pulsed mode-locked lasers combined with additional AOM frequency shifting methods.

[0055] In the process of implementing the concept of the present disclosure, the inventors discovered that there are at least the following problems in the related art:

[0056] Using the direct modulation method of a phase-type electro-optical modulator, if a spatial EOM based on a bulk optical crystal is used to modulate the laser, the frequency difference tuning range of the generated laser is limited due to the disadvantage of the narrow resonance bandwidth of this type of EOM; if a broadband fiber EOM is used to modulate the laser, a large range of frequency difference tuning can be achieved, but broadband fiber EOM usually operates in the infrared band, and the transition wavelength of the D1 line of most trapped particles belongs to the ultraviolet or blue light band. The multi-frequency coherent laser in the infrared band obtained by broadband fiber EOM modulation is difficult to drive the particles to achieve stimulated Raman transition.

[0057] Phase noise can be effectively eliminated by using the master-slave laser locking method, but this method requires an additional laser and the cost of setting up and maintaining the device is high.

[0058] The method of using a pulsed mode-locked laser combined with an additional AOM frequency shift requires a mode-locked laser that is expensive, and the system is complex and difficult to maintain. In addition, the transmission of mode-locked pulse light in the optical fiber is limited by the pulse broadening effect, which is not conducive to the realization of optical fiber integration of this method.

[0059] In a mixed cold atomic system, such as a mixed system of ytterbium ions and barium ions, to achieve simultaneous manipulation of the stimulated Raman transitions of several different particles with one laser, it is necessary to programmably control the frequency difference of multi-frequency coherent lasers.

[0060] In order to at least partially solve the technical problems existing in the related art, one aspect of the present disclosure provides a device for generating multi-frequency coherent laser, comprising:

[0061] Laser 1 is used to generate seed light, which is linearly polarized light with adjustable wavelength.

[0062] The signal generator 16 is used to generate a modulation signal 161 and a demodulation signal 162 with the same phase and the same frequency, wherein the signal frequencies of the modulation signal 161 and the demodulation signal 162 are adjustable.

[0063] The servo module 13 is used to generate a control signal 131 , and the voltage of the control signal 131 is adjustable.

[0064] The intensity modulation electro-optical modulator 4 is used to receive the modulation signal 161 and the control signal 131, and modulate the seed light to generate a first modulated optical signal, which includes a modulated sideband optical signal.

[0065] The polarization-maintaining optical fiber splitter 5 is used to split the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio, wherein the second modulated optical signal includes a modulated sideband optical signal.

[0066] The pre-processing module 19 is configured to receive the demodulated signal 162 and the feedback optical signal, generate an error signal so that the servo module 13 receives the error signal, and generate a control signal 131 based on the error signal.

[0067] The optical fiber amplifier 6 is used to amplify the power of the second modulated optical signal.

[0068] The periodically polarized crystal 8 is used to perform frequency doubling processing on the power-amplified second modulated optical signal to obtain multi-frequency coherent laser light, in which the frequency difference between two adjacent laser lights is twice the frequency of the radio frequency signal.

[0069] Figure 1 A block diagram of an apparatus for generating multi-frequency coherent laser light according to an embodiment of the present disclosure is schematically shown.

[0070] like Figure 1 As shown, the device for generating multi-frequency coherent laser according to this embodiment may include:

[0071] Laser 1 is used to generate seed light, which is linearly polarized light with adjustable wavelength.

[0072] More specifically, the wavelength of the seed light generated by the laser 1 may be in the near-infrared band. The seed light is stabilized by the laser in the laser 1 and then transmitted to the intensity-modulated electro-optical modulator 4 to be modulated.

[0073] The signal generator 16 generates a modulation signal 161 and a demodulation signal 162. The modulation signal 161 is sent to the intensity modulation electro-optical modulator 4 to participate in the modulation of the seed light, and the demodulation signal 162 is sent to the pre-processing module 19 to participate in the generation of the error signal.

[0074] More specifically, the signal generator 16 may also be a multi-channel synchronizable signal generator, and the signal generator 16 may directly generate the modulation signal 161 and the demodulation signal 162 with the same phase and frequency.

[0075] More specifically, the amplitude of modulation signal 161 is controlled within a preset range to meet modulation requirements. If the amplitude of modulation signal 161 is too small, the sideband components of the modulated optical signal generated after modulation will be weak in intensity, failing to meet the minimum input intensity requirement of fiber amplifier 6. If the amplitude of modulation signal 161 is too large, the modulated optical signal will have high-order sideband components, dispersing the energy of the modulated optical signal to these high-order sideband components, affecting the performance of the multi-frequency coherent laser.

[0076] More specifically, the frequency and phase of the modulation signal 161 and the demodulation signal 162 are the same. The amplitude of the demodulation signal 162 is within a reasonable range, so that the pre-processing module 19 can obtain a suitable amplitude of the error signal based on the demodulation signal 162.

[0077] The servo module 13 is used to generate a control signal 131. The amplitude of the control signal 131 changes according to the change of the intensity modulation type electro-optical modulator working point. The control signal enables the intensity modulation type electro-optical modulator 4 to stably operate at the minimum working point.

[0078] The intensity modulation electro-optical modulator 4 is used to receive the modulation signal 161 and the control signal 131, and modulate the seed light to generate a first modulated optical signal, which includes a modulated sideband optical signal.

[0079] More specifically, intensity-modulated electro-optical modulator 4 receives modulation signal 161, obtains a voltage corresponding to the amplitude of modulation signal 161, adjusts the frequency of the RF signal to half the corresponding particle energy level splitting frequency, and adjusts the amplitude of the RF signal to an appropriate value to ensure sufficient amplitude of the desired sidebands in the modulated optical signal while preventing the generation of higher-order sidebands. The AC signal amplitude that causes the phase of the modulated optical signal in intensity-modulated electro-optical modulator 4 to change by π is called the modulator's RF half-wave voltage.

[0080] More specifically, intensity-modulated electro-optical modulator 4 receives control signal 131 and obtains a voltage of control signal 131. Control signal 131 locks the operating point of the intensity-modulated electro-optical modulator to its minimum operating point. The amplitude of control signal 131 that causes the phase of the modulated optical signal in intensity-modulated electro-optical modulator 4 to change by π is called the modulator's bias half-wave voltage.

[0081] More specifically, intensity-modulated electro-optical modulator 4 modulates the seed light based on received modulation signal 161 and control signal 131 to generate a first modulated optical signal. The first modulated optical signal includes sideband light with a frequency midpoint equal to the frequency of modulation signal 161 and a frequency difference from the midpoint equal to the frequency of seed light.

[0082] The polarization-maintaining optical fiber splitter 5 is used to split the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio, wherein the second modulated optical signal includes a modulated sideband optical signal.

[0083] The preprocessing module 19 is used to receive the demodulated signal 162 and the above-mentioned feedback optical signal, generate an error signal so that the servo module 13 receives the error signal, and generate a control signal 131 based on the error signal. The amplitude of the modulation signal 131 changes according to the change of the operating point of the intensity modulation type electro-optical modulator 4. The modulation signal 131 enables the intensity modulation type electro-optical modulator 4 to stably operate at the minimum operating point.

[0084] More specifically, the servo module 13 can obtain the operating point drift of the intensity-modulated electro-optical modulator 4 based on the error signal, thereby adjusting the voltage of the generated control signal 131. The intensity-modulated electro-optical modulator 4 corrects and locks the operating point based on the control signal 131, so that the energy of the first modulated optical signal generated by the intensity-modulated electro-optical modulator 4 is concentrated in the two sideband lights.

[0085] The optical fiber amplifier 6 is used to amplify the power of the second modulated optical signal.

[0086] More specifically, the power of the second modulated optical signal obtained by beam splitting according to the preset power ratio can meet the minimum input optical power limit of the optical fiber amplifier 6, so that the optical fiber amplifier 6 can work normally and amplify the power of the second modulated optical signal.

[0087] The periodically polarized crystal 8 is used to perform frequency doubling processing on the power-amplified second modulated optical signal to obtain multi-frequency coherent laser light, in which the frequency difference between two adjacent laser lights is twice the frequency of the radio frequency signal.

[0088] More specifically, the multi-frequency coherent laser light obtained by the periodically poled crystal 8 is a fundamental wave having a frequency twice that of the seed light frequency, and two sidebands having a frequency difference of twice that of the radio frequency signal.

[0089] Reference below Figures 2 to 4 , combined with specific embodiments Figure 1 The device shown is further described.

[0090] Figure 2 FIG. 4 is a block diagram schematically illustrating an apparatus for generating multi-frequency coherent laser light according to another embodiment of the present disclosure.

[0091] like Figure 2 As shown, the seed light generated by laser 1 is linearly polarized light with adjustable wavelength. The polarization direction of the linearly polarized light meets the polarization direction requirement of the input of intensity-modulated electro-optical modulator 4. Various existing lasers can be used as laser 1 to achieve adjustable seed light wavelength. The seed light frequency generated by laser 1 is locked when generating multi-frequency coherent laser light.

[0092] The seed light is input into the intensity-modulated electro-optical modulator 4 through the polarization-maintaining optical fiber 2 , the first optical fiber coupling head and the connected polarization-maintaining optical fiber 3 .

[0093] The polarization-maintaining optical fiber 2 , the first optical fiber coupling head, and the connected polarization-maintaining optical fiber 3 can keep the polarization state of the seed light unchanged.

[0094] According to an embodiment of the present disclosure, the signal generator 16 may also be a single-channel synchronizable signal generator, and the signal generator 16 may directly generate a radio frequency signal, and use the power splitter 163 to generate a modulation signal 161 and a demodulation signal 162 with the same phase and frequency.

[0095] Modulation signal 161 is input via a coaxial cable to intensity-modulated electro-optical modulator 4, where it modulates the seed light and generates a first modulated optical signal. The generated first modulated optical signal includes sideband light with a frequency midpoint equal to the frequency of modulation signal 161 and a frequency difference from the midpoint equal to the frequency of the seed light. The frequency difference between the sideband lights of the first modulated optical signal can be adjusted by adjusting the frequency of the RF signal generated by signal generator 16.

[0096] According to an embodiment of the present disclosure, the amplitude of the modulated signal 161 needs to be controlled within a preset range. This can be achieved by controlling the power of the RF signal output by the signal generator 16. If the modulated signal power is too low, an RF amplifier can be set between the signal generator 16 and the power splitter 163 for adjustment. If the RF signal power is too high, an RF attenuator can be set between the signal generator 16 and the power splitter 163 for adjustment.

[0097] The modulated signal 161 is input to the intensity modulation type electro-optical modulator 4 via a coaxial cable.

[0098] The demodulated signal 162 is input to the pre-processing module 19 via a coaxial cable.

[0099] According to an embodiment of the present disclosure, the servo module 13 generates a voltage-adjustable control signal 131 , and the control signal 131 is input to the intensity-modulated electro-optical modulator 4 via a coaxial cable.

[0100] According to an embodiment of the present disclosure, the intensity modulated electro-optical modulator 4 may use a Mach-Zehnder interferometer structure, which includes two working arms of equal length, a radio frequency signal modulation port 401 and a control signal modulation port 402 .

[0101] The RF signal modulation port 401 is used to receive the modulation signal 161 , and the control signal modulation port 402 is used to receive the control signal 131 .

[0102] The two working arms of equal length change the optical path difference of the seed light transmitted on the two working arms of equal length based on the modulation signal 161 and the control signal 131 to generate a first modulated optical signal.

[0103] The Mach-Zehnder interferometer structure receives control signal 131 and operates at a minimum operating point. The Mach-Zehnder interferometer structure receives modulation signal 161 and modulates the seed light into a first modulated light signal in the minimum operating point state. The first modulated light signal includes sideband light having a frequency midpoint at the frequency of the seed light and a frequency difference from the frequency midpoint equal to the frequency of the modulation signal.

[0104] According to an embodiment of the present disclosure, the polarization-maintaining fiber splitter 5 includes an input optical fiber 501 , a second modulated optical signal output optical fiber 502 , and a feedback optical signal output optical fiber 503 .

[0105] More specifically, the input optical fiber 501 , the second modulated optical signal output optical fiber 502 , and the feedback optical signal output optical fiber 503 may be polarization-maintaining optical fibers.

[0106] The first modulated optical signal enters the polarization-maintaining optical fiber splitter 5 through the input optical fiber 501 . The polarization-maintaining optical fiber splitter 5 splits the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio.

[0107] Among them, the second modulated optical signal is input into the optical fiber amplifier 6 through the second modulated optical signal output end optical fiber 502. The second modulated optical signal is greater than the minimum input optical power required by the optical fiber amplifier 6 so that the optical fiber amplifier 6 can work normally and amplify the power of the second modulated optical signal.

[0108] The feedback optical signal is input to the pre-processing module 19 through the feedback optical signal output end optical fiber 503 . The feedback optical signal can be used to characterize the optical signal component of the second modulated optical signal.

[0109] According to an embodiment of the present disclosure, when the Mach-Zehnder interferometer structure operates at a minimum operating point, the first modulated optical signal outputted includes only the sideband light on both sides. However, during operation, the Mach-Zehnder interferometer structure may be affected by factors such as temperature, input light intensity, operating time, and / or static electricity, causing the minimum operating point to drift, thereby changing the optical path difference of the seed light transmitted on the two equal-length working arms. As a result, the first modulated optical signal generated by the output also includes a fundamental signal having a frequency equal to the frequency of the seed light.

[0110] According to an embodiment of the present disclosure, adjusting the voltage of the control signal 131 to stabilize the minimum operating point of the Mach-Zehnder interferometer structure can be achieved by:

[0111] According to an embodiment of the present disclosure, the preprocessing module 19 generates an error signal 132 based on the demodulated signal 162 and the feedback optical signal; the servo module 13 generates a control signal 131 based on the error signal 132, and the control signal 131 can make the Mach-Zehnder interferometer structure stably operate at the minimum operating point.

[0112] According to an embodiment of the present disclosure, the pre-processing module 19 includes:

[0113] The photodetector 14 is used to receive the feedback optical signal and generate a feedback electrical signal.

[0114] More specifically, the feedback optical signal is input to the photodetector 14 in a fiber-optic coupling manner. The photodetector 14 converts the feedback optical signal into a feedback electrical signal. The voltage of the feedback electrical signal is proportional to the light intensity of the feedback optical signal.

[0115] The microwave phase shifter 18 is used to perform phase shifting on the feedback electrical signal to generate a phase-shifted feedback electrical signal.

[0116] The mixer 17 is used to mix the feedback electrical signal and the demodulation signal 162 to generate a mixed signal.

[0117] The low-pass filter 12 is used to filter the mixing signal to generate an error signal 132. The error signal 132 is used to represent the optical path difference of the seed light transmitted on the two equal-length working arms of the Mach-Zehnder interferometer structure.

[0118] More specifically, the microwave phase shifter 18 adjusts the phase of the feedback electrical signal generated by the photodetector 14 so that the signal envelope of the error signal 132 generated after mixing and filtering the phase-shifted feedback electrical signal is symmetrical about the center of the signal amplitude.

[0119] More specifically, each part in the pre-processing module 19 transmits signals via a coaxial cable.

[0120] The error signal 132 is input to the servo module 13 via a coaxial cable.

[0121] According to an embodiment of the present disclosure, the servo module 13 generates a control signal 131 through a PID algorithm based on the error signal 132. The voltage of the control signal 131 changes with the change of the working point of the Mach-Zehnder interferometer, so that the Mach-Zehnder interferometer structure can stably operate at the minimum working point.

[0122] More specifically, after receiving the error signal 132, the servo module 13 can obtain optical signal component information of the second modulated optical signal, thereby obtaining real-time information about the optical path difference of the seed light transmitted on the two working arms when the Mach-Zehnder interferometer structure is operating. The control signal 131 generated by the PID algorithm can adjust the optical path difference of the seed light transmitted on the two working arms when the Mach-Zehnder interferometer structure is operating, so that the seed light transmitted on the two working arms maintains a phase difference of π, thereby achieving stable operation of the Mach-Zehnder interferometer structure at the minimum operating point.

[0123] Figure 3 The figure schematically shows a frequency component diagram contained in the first modulated optical signal output by the intensity modulation electro-optical modulator 4 according to an embodiment of the present disclosure.

[0124] like Figure 3 As shown, the first modulated optical signal includes two sideband components, ω0-ω1 and ω0+ω1, generated by seed light modulated by the Mach-Zehnder interferometer structure. ω0 is the frequency of the seed light, i.e., the frequency of the carrier component; ω1 is the frequency of the modulation signal 161 applied to the RF signal modulation port 401. Modulation signal 161 is generated by signal generator 16 and is half the energy level splitting frequency of the corresponding particle undergoing stimulated Raman transition using the multi-frequency coherent laser.

[0125] According to an embodiment of the present disclosure, the Mach-Zehnder interferometer structure operates at different operating points based on the voltage applied to the control signal modulation port 402. If the applied voltage maximizes the modulator output and the light field interference of the seed light on the two working arms is constructive, it is called the maximum operating point; if the applied voltage minimizes the modulator output and the light field interference of the seed light on the two working arms is destructive, it is called the minimum operating point. When the Mach-Zehnder interferometer structure operates at the minimum operating point, the modulation signal modulation port 401 receives the modulation signal 161, and the control signal modulation port 402 receives the control signal 131, so that the light field of the first modulated light signal output by the Mach-Zehnder interferometer structure contains only sideband components, and the carrier component is suppressed.

[0126] According to an embodiment of the present disclosure, the first modulated optical signal is split into a second modulated optical signal and a feedback optical signal according to a preset power ratio by a polarization-maintaining optical fiber splitter 5. The second modulated optical signal is amplified by an optical fiber amplifier 6 and then input into a periodically polarized crystal 8 for frequency doubling to obtain a multi-frequency coherent laser. The multi-frequency coherent laser has a fundamental wave with a frequency twice that of the seed light frequency and two sidebands with a frequency difference of twice the frequency of the radio frequency signal. The frequency difference of the multi-frequency coherent laser can be adjusted by adjusting the frequency of the radio frequency signal generated by the signal generator 16.

[0127] According to an embodiment of the present disclosure, the fiber amplifier 6 includes a controller and a laser output head, which are connected by a power-transmitting optical fiber. The controller includes a laser input head, which receives a second modulated optical signal. The fiber amplifier 6 amplifies the power of the second modulated optical signal, and the laser output head outputs the power-amplified second modulated optical signal as spatial light. The fiber amplifier 6 has a minimum input optical power limit. Connecting the second modulated optical signal output end optical fiber 502 to the laser input head of the fiber amplifier 6 ensures that the power of the second modulated optical signal exceeds the minimum input optical power limit of the fiber amplifier 6, enabling the fiber amplifier 6 to operate normally.

[0128] According to the disclosed embodiment, a focusing lens 7 is disposed between the fiber amplifier 6 and the periodically poled crystal 8. When the Mach-Zehnder interferometer structure operates stably at its minimum operating point, the optical field of the second modulated optical signal input to the fiber amplifier 6 contains only two sideband frequency components. The fiber amplifier 6 outputs the amplified second modulated optical signal as spatial light. Due to the small end face area of ​​the periodically poled crystal 8, the use of the focusing lens 7 ensures that the entire amplified second modulated optical signal enters the periodically poled crystal 8.

[0129] According to an embodiment of the present disclosure, the periodically poled crystal 8 may be a periodically poled lithium niobate (PPLN) crystal or a periodically poled lithium tantalate (PPSLT) crystal.

[0130] According to the embodiments of the present disclosure, the frequency-doubling efficiency of the periodically poled crystal 8 is related to the temperature and the optical field frequency of the input optical signal. When the input optical field frequency is constant, the crystal temperature can be adjusted by providing a temperature adjustment device to achieve the maximum frequency-doubling efficiency. The temperature adjustment device is also used to stabilize the frequency-doubling efficiency of the periodically poled crystal 8.

[0131] According to the embodiments of the present disclosure, the periodically poled crystal 8 has high frequency doubling efficiency. Only a single pass of the second modulated optical signal through the periodically poled crystal 8 is required to produce a strong multi-frequency coherent laser. Frequency doubling does not require the use of an optical resonant cavity and is therefore not limited by the peak width of the optical resonant cavity. The bandwidth of the periodically poled crystal 8 is adjustable, and the adjustable range is greater than that of frequency doubling using an optical resonant cavity.

[0132] According to the embodiments of the present disclosure, existing intensity-modulated electro-optical modulators mostly operate in the infrared or near-infrared bands, while the D1 line transition bands of most particles capable of stimulated Raman transitions correspond to the ultraviolet or blue light bands, which are difficult to access using existing intensity-modulated electro-optical modulators. The embodiments of the present disclosure utilize a periodically poled crystal 8 to convert infrared or near-infrared light into ultraviolet or blue light through a second harmonic generation (or frequency doubling) process.

[0133] According to an embodiment of the present disclosure, a collimating lens 9 is provided at the other end of the periodically poled crystal 8 relative to the optical fiber amplifier 6 , and the collimating lens 9 is used to collimate the multi-frequency coherent laser.

[0134] According to the embodiments of the present disclosure, the spatial optical path of the multi-frequency coherent laser generated by the periodically polarized frequency-doubling crystal 8 only requires lens focusing. No reflectors are involved in the generation of the multi-frequency coherent laser, thereby avoiding errors caused by the jitter of the reflectors during the use of the device. The device has the same stabilizing effect as the laser 1, so the power and direction of the multi-frequency coherent laser are very stable.

[0135] According to the embodiment of the present disclosure, based on the operating bandwidth of the intensity-modulated electro-optic modulator 4 and the periodically poled crystal 8, the frequency difference of the generated multi-frequency coherent laser light can range from several GHz to 80 GHz.

[0136] Figure 4 The figure schematically shows the frequency components contained in the multi-frequency coherent laser output by the periodically poled crystal 8 according to the embodiment of the present disclosure.

[0137] like Figure 4 As shown, after the two sideband components in the second modulated optical signal undergo the second harmonic process, the sideband component with a frequency of ω0-ω1 is converted into a sideband component with a frequency of 2ω0-2ω1, and the sideband component with a frequency of ω0+ω1 is converted into a sideband component with a frequency of 2ω0+2ω1. During the second harmonic process, the combination of the above two sideband components will also produce a fundamental component with a frequency of 2ω0.

[0138] According to an embodiment of the present disclosure, an optical beam splitter 10 may be further provided after the collimating lens 9. After passing through the optical beam splitter 10, the multi-frequency coherent laser light is split into an experimental multi-frequency coherent laser light along the propagation direction of the multi-frequency coherent laser light and an observation multi-frequency coherent laser light perpendicular to the propagation direction of the multi-frequency coherent laser light. The experimental multi-frequency coherent laser light is transmitted through a second fiber coupler and a connected polarization-maintaining fiber 11; the observation multi-frequency coherent laser light is transmitted through a third fiber coupler and a connected polarization-maintaining fiber 15, so that the frequency components of the observation multi-frequency coherent laser light can be analyzed.

[0139] According to the embodiments of the present disclosure, the sideband components with a frequency of 2ω0-2ω1 and a sideband component with a frequency of 2ω0+2ω1 are actually driven by multi-frequency coherent laser to stimulate Raman transitions. To ensure the maximum interaction intensity, in order to make the multi-frequency coherent laser focus on the required sideband components, it is necessary to make the Mach-Zehnder interferometer structure operate stably at the minimum operating point when modulating the seed light, so as to obtain a first modulated optical signal that only includes sideband components with frequencies of ω0-ω1 and ω0+ω1.

[0140] According to the embodiment of the present disclosure, the optical fiber amplifier 6 is selected based on the wavelength required for the stimulated Raman transition of the corresponding particles using multi-frequency coherent lasers to perform stimulated Raman transitions. Other types of optical fiber amplifiers can be configured as needed.

[0141] According to an embodiment of the present disclosure, the laser 1 may also be replaced by other types of lasers, as long as the optical power output by the replacement laser can provide sufficient interaction intensity.

[0142] According to an embodiment of the present disclosure, for a device for generating multi-frequency coherent lasers that drive stimulated Raman transitions of ytterbium atoms, a laser with a 1064nm seed light can be used as the laser 1, and a first polarization-maintaining optical fiber and a coupling head 3 corresponding to a wavelength of 1064nm, an intensity-modulated electro-optical modulator 4, a polarization-maintaining optical fiber beam splitter 5, a focusing lens 7, and a periodically polarized crystal 8 are used; a collimating lens 9, a second polarization-maintaining optical fiber and a coupling head 11, and a third polarization-maintaining optical fiber and a coupling head 15 corresponding to half the wavelength of the seed light, that is, a wavelength of 532nm are used; an ytterbium-doped fiber amplifier is used as the fiber amplifier 6, and the ytterbium-doped fiber amplifier operates in the infrared band, with a specific operating wavelength of 976nm to 1120nm, and the linewidth of the output laser is in the kHz range, with a polarization degree greater than 23dB, and an optical power of more than 50W can be obtained at the maximum operating current of the amplifier; and the output frequency of the signal generator 16 is set to 6.32GHz.

[0143] According to an embodiment of the present disclosure, for a device for generating multi-frequency coherent lasers that drive stimulated Raman transitions of barium atoms, a laser with a 1064nm seed light can be used as the laser 1, and a first polarization-maintaining optical fiber and a coupling head 3 corresponding to a wavelength of 1064nm, an intensity-modulated electro-optical modulator 4, a polarization-maintaining optical fiber beam splitter 5, a focusing lens 7, and a periodically polarized crystal 8 are used; a collimating lens 9, a second polarization-maintaining optical fiber and a coupling head 11, and a third polarization-maintaining optical fiber and a coupling head 15 corresponding to half the wavelength of the seed light, that is, a wavelength of 532nm are used; an ytterbium-doped fiber amplifier is used as the fiber amplifier 6, and the ytterbium-doped fiber amplifier operates in the infrared band, with a specific operating wavelength of 976nm to 1120nm, and the linewidth of the output laser is in the kHz range, with a polarization degree greater than 23dB, and an optical power of more than 50W can be obtained at the maximum operating current of the amplifier; and the output frequency of the signal generator 16 is set to 4.02GHz.

[0144] According to an embodiment of the present disclosure, for a device for generating multi-frequency coherent lasers that drive stimulated Raman transitions of rubidium atoms, a laser with a seed light of 1560nm can be used as the laser 1, and a first polarization-maintaining optical fiber and a coupling head 3 corresponding to a wavelength of 1560nm, an intensity-modulated electro-optical modulator 4, a polarization-maintaining optical fiber beam splitter 5, a focusing lens 7, and a periodically polarized crystal 8 are used; a collimating lens 9, a second polarization-maintaining optical fiber and a coupling head 11, and a third polarization-maintaining optical fiber and a coupling head 15 corresponding to half the wavelength of the seed light, that is, a wavelength of 780nm are used; an erbium-doped fiber amplifier is used as the optical fiber amplifier 6, and the ytterbium-doped fiber amplifier operates in the infrared band, with a specific operating wavelength of 1530nm to 1598nm. The linewidth of the output laser is in the kHz range, with a polarization degree greater than 20dB, and an optical power of more than 15W can be obtained at the maximum operating current of the amplifier; and the output frequency of the signal generator 16 is set to 3.417GHz.

[0145] According to the embodiments of the present disclosure, the device has low device cost and does not include a reflector module, that is, no unnecessary jitter will be generated during use, so that the beam power and pointing are stable; by adjusting the output values ​​of the laser 1, the signal generator 16 and the servo module 13, the driving frequency of the electro-optical modulator can be flexibly adjusted to obtain a multi-frequency coherent laser suitable for stimulated Raman transitions of various particles.

[0146] According to an embodiment of the present disclosure, a device for generating frequency-difference programmable multi-frequency coherent laser is provided, which can operate in the visible and infrared bands, has a large modulation bandwidth, is easy to maintain, has high stability, is low in cost, and is fiber-compatible. The multi-frequency coherent laser generated by the device is suitable for ion-atom mixing systems or heterogeneous ion mixing systems, that is, the multi-frequency coherent laser can be used to simultaneously perform coherent manipulation of the quantum states of ions and atoms or heterogeneous ions.

[0147] Another aspect of the present disclosure provides a method for generating multi-frequency coherent laser light, comprising:

[0148] The laser 2 is used to generate seed light, which is linearly polarized light with adjustable wavelength.

[0149] The signal generator 16 is used to generate a modulation signal 161 and a demodulation signal 162 ; the frequency of the modulation signal 161 is equal to the frequency of the demodulation signal 162 , and the frequencies of the modulation signal 161 and the demodulation signal 162 are adjustable.

[0150] The servo module 13 is used to generate a control signal 131 , and the voltage of the control signal 131 is adjustable.

[0151] The intensity modulation type electro-optical modulator 4 receives the modulation signal 161 and the control signal 131 and modulates the seed light to generate a first modulated optical signal. The first modulated optical signal includes a modulated sideband optical signal.

[0152] The polarization-maintaining optical fiber splitter 5 is used to split the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio, wherein the second modulated optical signal includes a modulated sideband optical signal.

[0153] The demodulated signal and the feedback optical signal are received 162 by the pre-processing module 19 to generate an error signal 132 . The servo module 13 receives the error signal 132 and generates a control signal 131 based on the error signal 132 .

[0154] The optical fiber amplifier 6 is used to amplify the power of the second modulated optical signal.

[0155] The power-amplified second modulated optical signal is frequency-doubled using a periodically polarized crystal 8 to obtain a multi-frequency coherent laser. The frequency difference between two adjacent lasers in the multi-frequency coherent laser is twice the frequency of the radio frequency signal, that is, the frequency difference between two adjacent lasers in the multi-frequency coherent laser is the energy level splitting frequency of the corresponding particles that undergo stimulated Raman transition using the multi-frequency coherent laser.

[0156] According to an embodiment of the present disclosure, the servo module 13 receives the error signal 132 and generates the control signal 131 based on the error signal 132, including:

[0157] The servo module 13 generates a control signal 131 through a PID algorithm based on the error signal 132 generated by the preprocessing module 19 . The voltage of the control signal 131 is a bias half-wave voltage that enables the Mach-Zehnder interferometer structure to operate at the minimum operating point.

[0158] According to an embodiment of the present disclosure, a method for generating multi-frequency coherent laser utilizes intensity modulation and frequency doubling processes to generate multi-frequency coherent laser, which can obtain spectral components that only include the required sideband signals, avoids the problem of laser coherence decomposition caused by excessive spectral components, and can improve the interaction intensity.

[0159] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present disclosure. It should be understood that the above are only specific embodiments of the present disclosure and are not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. A device for generating multi-frequency coherent laser light, comprising: A laser for generating seed light, wherein the seed light is linearly polarized light with adjustable wavelength; A signal generator, configured to generate a modulation signal and a demodulation signal with the same phase and frequency, wherein the signal frequencies of the modulation signal and the demodulation signal are adjustable; A servo module is used to generate a control signal, wherein the amplitude of the control signal is adjustable; an intensity modulation electro-optical modulator, which modulates the seed light based on the received modulation signal and the control signal to generate a first modulated optical signal, wherein the first modulated optical signal includes a modulated sideband optical signal; a polarization-maintaining optical fiber splitter, configured to split the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio, wherein the second modulated optical signal includes a modulated sideband optical signal; a pre-processing module, configured to receive the demodulated signal and the feedback optical signal, and generate an error signal by frequency mixing and phase shifting, so that the servo module receives the error signal and generates the control signal based on the error signal; an optical fiber amplifier, configured to amplify the power of the second modulated optical signal; The periodically polarized crystal is used to perform frequency doubling processing on the power-amplified second modulated optical signal to obtain multi-frequency coherent laser light, wherein the frequency difference between two adjacent lasers in the multi-frequency coherent laser light is twice the frequency of the radio frequency signal.

2. The device according to claim 1, wherein The intensity modulated electro-optical modulator uses a Mach-Zehnder interferometer structure, which includes two working arms of equal length, a radio frequency signal modulation port, and a control signal modulation port; The RF signal modulation port is used to receive the modulation signal, and the control signal modulation port is used to receive the control signal; The two working arms of equal length change the optical path difference of the seed light transmitted on the two working arms of equal length based on the modulation signal and the control signal to generate the first modulated optical signal.

3. The device according to claim 1, wherein The frequencies of the modulation signal and the demodulation signal generated by the signal generator are half of the energy level splitting frequency of the corresponding particles that undergo stimulated Raman transition using the multi-frequency coherent laser.

4. The device according to claim 2, wherein The voltage of the control signal generated by the servo module is a bias half-wave voltage that enables the Mach-Zehnder interferometer structure to operate at a minimum operating point.

5. The device according to claim 2, wherein The pre-processing module comprises: a photoelectric detector, configured to receive the feedback optical signal and generate a feedback electrical signal; a microwave phase shifter, configured to perform phase shifting on the feedback electrical signal to generate a phase-shifted feedback electrical signal; a mixer, configured to mix the feedback electrical signal and the demodulated signal to generate a mixed signal; A low-pass filter is used to filter the mixing signal to generate an error signal, wherein the error signal is used to characterize the optical path difference of the seed light transmitted on two equal-length working arms of the Mach-Zehnder interferometer structure.

6. The apparatus of claim 1 , further comprising: a focusing lens, disposed between the optical fiber amplifier and the periodically poled crystal, the focusing lens being used to control the entire second modulated optical signal to enter the periodically poled crystal; A collimating lens is provided at the other end of the periodically polarized crystal relative to the optical fiber amplifier, and the collimating lens is used to collimate the multi-frequency coherent laser.

7. The device according to claim 1, wherein Polarization-maintaining optical fibers are used to connect the laser and the intensity-modulated electro-optical modulator, the intensity-modulated electro-optical modulator and the polarization-maintaining optical fiber splitter, the polarization-maintaining optical fiber splitter and the optical fiber amplifier, and the polarization-maintaining optical fiber splitter and the preprocessing module.

8. The device according to claim 1, wherein The signal generator and the intensity modulation electro-optical modulator, the signal generator and the pre-processing module, the pre-processing module and the servo module, and the servo module and the intensity modulation electro-optical modulator are connected using coaxial cables.

9. A method for generating multi-frequency coherent laser light, comprising: A laser is used to generate seed light, wherein the seed light is linearly polarized light with adjustable wavelength; A signal generator is used to generate a modulation signal and a demodulation signal with the same phase and the same frequency, wherein the signal frequencies of the modulation signal and the demodulation signal are adjustable; A servo module is used to generate a control signal, wherein the amplitude of the control signal is adjustable; Using an intensity modulation electro-optical modulator to receive the modulation signal and the control signal, and modulate the seed light to generate a first modulated optical signal, wherein the first modulated optical signal includes a modulated sideband optical signal; Using a polarization-maintaining fiber splitter to split the first modulated optical signal into a second modulated optical signal and a feedback optical signal according to a preset power ratio, wherein the second modulated optical signal includes a modulated sideband optical signal; Using a pre-processing module to receive the demodulated signal and the feedback optical signal, and generate an error signal through frequency mixing and phase modulation, so that the servo module receives the error signal and generates the control signal based on the error signal; amplifying the power of the second modulated optical signal using an optical fiber amplifier; The power-amplified second modulated optical signal is frequency-doubled by using a periodically polarized crystal to obtain a multi-frequency coherent laser, wherein the frequency difference between two adjacent lasers in the multi-frequency coherent laser is twice the frequency of the radio frequency signal.

10. The method of claim 9, wherein: The servo module receives the error signal, and generates the control signal based on the error signal, comprising: The servo module generates the control signal through a PID algorithm based on the error signal generated by the preprocessing module. The voltage of the control signal is a bias half-wave voltage that enables the Mach-Zehnder interferometer structure to operate at a minimum operating point.

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