Optical isolator and light source apparatus

Abstract

The optical isolator comprises a polarization control component and an atomic cell. The polarization control component is suitable for changing the polarization state of an initial laser beam generated by a laser, and outputting circularly polarized light comprising right-handed circularly polarized light and left-handed circularly polarized light. The atomic cell is internally provided with an atomic ensemble, and the atomic ensemble is polarized by the circularly polarized light. Part of the circularly polarized light output by the atomic ensemble will return along the original path, forming reverse circularly polarized light comprising reverse left-handed circularly polarized light and reverse right-handed circularly polarized light. The reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarization control component, so as to achieve isolation of the reverse circularly polarized light and prevent the reverse circularly polarized light from being incident on the laser.

Description

TECHNICAL FIELD

[0001] At least one embodiment of the present disclosure relates to the technical field of laser isolators, and more particularly to an optical isolator and a light source device. BACKGROUND

[0002] Laser, as the "eyes" and "hands" of physicists interacting with the microscopic world, has played an indispensable role in the evolution of modern physics, and has promoted the development of multiple fields including optics, atomic and molecular physics, quantum information, etc. In addition, laser is also an important technology in modern engineering field, and its applications include laser processing, laser imaging, laser radar, etc. Generally, lasers need to be used with optical isolators, which can ensure that the laser generated by the laser passes through while preventing the reflected laser from entering the laser. When the reflected light enters the laser, the reflected noise will also be amplified inside the laser, affecting the mode and frequency stability of the laser, so the optical isolator plays an important role in various precision optical experiments.

[0003] Faraday isolator is the most widely used isolator at present, and the commercial Faraday isolator technology is very mature. It has the advantages of large bandwidth (about 10 nm), high isolation ratio (about 30 dB for single-stage isolator, about 50 dB for cascaded isolator) and low insertion loss (about 1 dB), and can realize the isolation of single photon to continuous wave and pulsed laser. The main disadvantage of Faraday isolator is that it requires a certain strong magnetic field (usually about 1 T) to realize Faraday magneto-optical effect, which will have a greater impact on other optical devices nearby, and it is difficult to integrate the magneto-optical medium.

[0004] Optical isolators are roughly divided into two types. The first type of optical isolator is to apply directional spatiotemporal modulation on the optical medium, and to realize the difference between forward and reverse transmission through the directionality of the signal light on the frequency conversion process of the nonlinear medium. This selectivity comes from the phase matching condition in the frequency conversion process. The optical isolation is realized based on acoustic modulation, microwave modulation and optical modulation. The defect is that additional resources need to be invested to realize the modulation of the medium, and there is also a challenge of separating the signal light from other stray light. The second type of optical isolator is to combine Kerr nonlinear effect and asymmetric structure to realize non-reciprocity, which can realize a passive optical isolator without magnetic field and without any external driving. However, the disadvantage is that it cannot simultaneously realize the transmission of forward light and the isolation of reverse light.

[0005] Therefore, the optical isolator in the related art cannot simultaneously realize the transmission of forward light and the isolation of reverse light without applying external driving. SUMMARY

[0006] To solve the above and other problems in the prior art, the present disclosure provides an optical isolator capable of realizing the passing of forward light and the isolation of reverse light without applying external driving.

[0007] According to a first aspect of the present disclosure, an optical isolator is provided, comprising:

[0008] a polarization control component adapted to change the polarization state of an initial laser beam generated by a laser, outputting circularly polarized light, the circularly polarized light including right-handed circularly polarized light and left-handed circularly polarized light; and

[0009] an atomic cell, the atomic cell being internally provided with an atomic ensemble, the atomic ensemble being polarized by the circularly polarized light;

[0010] wherein part of the circularly polarized light output by the atomic ensemble will return along the original path, forming reverse circularly polarized light, the reverse circularly polarized light including reverse left-handed circularly polarized light and reverse right-handed circularly polarized light; the reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarization control component, so as to realize the isolation of the reverse circularly polarized light and prevent the reverse circularly polarized light from being incident on the laser.

[0011] According to an embodiment of the present disclosure, the atomic ensemble and the circularly polarized light have a self-induced non-reciprocal phenomenon, that is, after the atomic ensemble is polarized by the circularly polarized light, the circularly polarized light and the reverse circularly polarized light will produce different phases and losses when transmitting in opposite directions in the atomic ensemble, so as to realize the non-reciprocal transmission of the circularly polarized light and the reverse circularly polarized light.

[0012] According to an embodiment of the present disclosure, the polarization control component comprises:

[0013] a polarizer adapted to change the initial laser beam into linearly polarized light;

[0014] a quarter-wave plate adapted to change the polarization state of the linearly polarized light, outputting first polarized light; and

[0015] a half-wave plate adapted to change the polarization state of the first polarized light, outputting the circularly polarized light;

[0016] wherein the angles of the polarizer, the first wave plate and the second wave plate are rotated to improve the purity of the circularly polarized light.

[0017] According to an embodiment of the present disclosure, the atomic ensemble comprises atoms having a specific transition, wherein the specific transition is a transition from a lower energy level with angular momentum L to an upper energy level with angular momentum L-1 or L.

[0018] According to an embodiment of the present disclosure, the atomic ensemble further comprises a buffer gas, and the buffer gas is adapted to increase an absorption linewidth of the atomic ensemble to the counter-clockwise circularly polarized light, so as to increase a working bandwidth of the optical isolator.

[0019] According to an embodiment of the present disclosure, a heating belt and a sheet-shaped permalloy are arranged outside the atomic cell,

[0020] The heating belt is adapted to increase a temperature of the atomic ensemble, so as to increase a density of the atomic ensemble, and further increase an absorption of the atomic ensemble to the counter-clockwise circularly polarized light.

[0021] The sheet-shaped permalloy is adapted to shield an external magnetic field, so as to prevent the external magnetic field from destroying a polarization process of the atomic ensemble to the circularly polarized light.

[0022] According to a second aspect of the present disclosure, an optical isolator is provided, comprising:

[0023] a single-mode waveguide adapted to transmit an initial laser beam generated by a laser and form an evanescent field on a surface of the single-mode waveguide; and

[0024] an optical medium arranged adjacent to the single-mode waveguide, so that the optical medium can be polarized by the evanescent field;

[0025] wherein part of the initial laser beam is reflected to form a first laser beam, the first laser beam is transmitted in a reverse direction along the single-mode waveguide, and a reverse evanescent field is formed on the surface of the single-mode waveguide, a main circularly polarized light component of the reverse evanescent field is absorbed by the optical medium, so as to achieve isolation of the first laser beam and prevent the first laser beam from being incident on the laser.

[0026] According to an embodiment of the present disclosure, the optical isolator is integrated on a photonic integrated chip.

[0027] According to a third aspect of the present disclosure, a light source device is provided, comprising:

[0028] a laser adapted to provide an initial laser beam;

[0029] the optical isolator is adapted to allow the initial laser beam to pass in a forward direction, and prevent part of the initial laser beam returning along the original path from being incident on the laser.

[0030] According to an embodiment of the present disclosure, the light source device further comprises:

[0031] a polarization analyzer adapted to monitor a polarization state of the initial laser beam output by the optical isolator, so as to adjust a spatial position of the optical isolator in real time, and further improve a purity of a laser beam output by the optical isolator.

[0032] According to the embodiment of the present disclosure, by setting the polarization control component to change the polarization state of the initial laser beam generated by the laser, the atomic ensemble inside the atomic cell is polarized by the circularly polarized light; part of the circularly polarized light output by the atomic ensemble will return along the original path, forming reverse circularly polarized light, which includes reverse left-handed circularly polarized light and reverse right-handed circularly polarized light; the reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarization control component, achieving isolation of the reverse circularly polarized light and preventing the reverse circularly polarized light from being incident on the laser, thereby achieving the isolation of the forward light (i.e. circularly polarized light) and the reverse light (i.e. reverse circularly polarized light) without applying external driving. BRIEF DESCRIPTION OF DRAWINGS

[0033] The above and other objects, features and advantages of the present disclosure will become more apparent from the following description of embodiments of the present disclosure, taken in conjunction with the accompanying drawings, in which:

[0034] Figure 1 A working principle diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown;

[0035] Figure 2 A simple schematic diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown;

[0036] Figure 3 A schematic diagram of a rubidium 87 atomic D1 transition energy level according to an embodiment of the present disclosure is schematically shown;

[0037] Figure 4 A polarization transformation schematic diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown;

[0038] Figure 5 An isolation effect test optical path diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown;

[0039] Figure 6 A reverse light transmittance measurement result diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown; and

[0040] Figure 7 A working principle diagram of an optical isolator according to another embodiment of the present disclosure is schematically shown.

[0041] In the above drawings, the meanings of the reference signs are as follows:

[0042] 1 - laser;

[0043] 2 - polarization control component;

[0044] 21 - polarizer;

[0045] 22 - quarter-wave plate;

[0046] 23 - Half-wave plate;

[0047] 3 - Atomic gas cell;

[0048] 4 - Single-mode waveguide;

[0049] 5 - Optical medium;

[0050] 6 - Protective layer;

[0051] 7 - Chip substrate;

[0052] 8 - Acousto-optic modulator;

[0053] 9 - Chopper;

[0054] 10 - Beam splitter;

[0055] 11 - Photodetector. DETAILED DESCRIPTION

[0056] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. It should be understood, however, that the description which follows is merely illustrative and is not intended to limit the scope of the present disclosure. In the following detailed description of embodiments of the present disclosure, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it would be apparent to one skilled in the art that the embodiments of the present disclosure can be practiced without these specific details. In other instances, well-known structures and

[0057] The terms used herein are merely used to describe specific embodiments and are not intended to limit the present disclosure. The terms "include", "comprise" and the like used herein indicate the presence of the described features, steps, operations, and / or components but do not preclude the presence or addition of one or more other features, steps, operations, or components.

[0058] All terms used herein, including technical and scientific terms, have the same meanings as those generally understood by those skilled in the art unless otherwise defined. It should be noted that the terms used herein should be interpreted as having meanings that are consistent with the context of the present description, and should not be interpreted in an idealized or excessively formal manner.

[0059] In the case of using expressions similar to "at least one of A, B, and C, etc.", it should generally be interpreted to include at least one of each item enumerated, unless otherwise defined. For example, "a system having at least one of A, B, and C" should be interpreted to include a system having at least one of A, a system having at least one of B, a system having at least one of C, a system having at least one of A and B, a system having at least one of A and C, a system having at least one of B and C, and / or a system having at least one of A, B, and C, etc.

[0060] Optical isolators are roughly divided into two types. The first type of optical isolator is to apply directional spatiotemporal modulation on an optical medium, to realize the difference between forward and reverse transmission through the direction selectivity of the signal light in the frequency conversion process on the nonlinear medium. The selectivity is derived from the phase matching condition in the frequency conversion process. The optical isolation is realized based on acoustic modulation, microwave modulation, and optical modulation. The defect is that additional resources are needed to modulate the medium, and there is also a challenge to separate the signal light from other stray light. The second type of optical isolator is to combine Kerr nonlinear effect and asymmetric structure to realize non-reciprocity, which can realize a passive optical isolator without magnetic field and without any external driving. However, the defect is that it cannot simultaneously realize the passing of forward light and the isolation of reverse light. The optical isolator in the related art cannot simultaneously realize the passing of forward light and the isolation of reverse light without applying external driving.

[0061] Therefore, the present disclosure provides an optical isolator. A polarization control component is arranged to change the polarization state of an initial laser beam generated by a laser, to output circularly polarized light. An atomic ensemble in an atomic cell is polarized by the circularly polarized light. Part of the circularly polarized light output by the atomic ensemble will return along the original path to form reverse circularly polarized light, which includes reverse left-handed circularly polarized light and reverse right-handed circularly polarized light. The reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarization control component, to realize the isolation of the reverse circularly polarized light and prevent the reverse circularly polarized light from being incident on the laser. Thus, the passing of forward light (i.e., the circularly polarized light) and the isolation of reverse light (i.e., the reverse circularly polarized light) are realized without applying external driving.

[0062] Figure 1 A working principle diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown.

[0063] According to some embodiments of the present disclosure, as shown in Figure 1 The polarization control component 2 is adapted to change the polarization state of an initial laser beam generated by a laser, to output circularly polarized light, which includes right-handed circularly polarized light and left-handed circularly polarized light. The atomic ensemble in the atomic cell is polarized by the circularly polarized light.

[0064] According to some embodiments of the present disclosure, part of the circularly polarized light output by the atomic ensemble will return along the original path to form reverse circularly polarized light, which includes reverse left-handed circularly polarized light and reverse right-handed circularly polarized light. The reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarization control component 2, to realize the isolation of the reverse circularly polarized light and prevent the reverse circularly polarized light from being incident on the laser 1.

[0065] According to some embodiments of this disclosure, by setting a polarization control component 2 to change the polarization state of the initial laser beam generated by the laser 1, circularly polarized light is output, and the atomic ensemble inside the atomic gas chamber 3 is polarized by the circularly polarized light; part of the circularly polarized light output from the atomic ensemble will return along the original path to form reverse circularly polarized light, which includes reverse left-handed circularly polarized light and reverse right-handed circularly polarized light; the reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarization control component 2, thereby achieving isolation of the reverse circularly polarized light and preventing it from incident on the laser 1. This achieves the isolation of forward light (i.e., circularly polarized light) from reverse light (i.e., reverse circularly polarized light) without a magnetic field or any external driving force. In particular, the optical isolator of the embodiments of this disclosure does not require the setting of a magnetic optical isolator during the absorption or isolation of reverse circularly polarized light, and is therefore also called a non-magnetic optical isolator.

[0066] According to some embodiments of this disclosure, an atomic ensemble is a system composed of a large number of atoms that exhibit statistical regularities on a macroscopic level, while each has a different state on a microscopic level.

[0067] According to some embodiments of this disclosure, there is a self-induced non-reciprocity phenomenon between the atomic ensemble and circularly polarized light. The self-induced non-reciprocity is that after the atomic ensemble is polarized by circularly polarized light, the circularly polarized light and the reverse circularly polarized light will produce different phases and losses when propagating in opposite directions in the atomic ensemble, so as to achieve non-reciprocal propagation of circularly polarized light and reverse circularly polarized light.

[0068] According to some embodiments of this disclosure, self-induced non-reciprocity is not limited to specific transitions of a particular atom, but can be used in various atoms, molecules, and doped media. Self-induced non-reciprocity means that after a circularly polarized light field polarizes a medium, the medium produces different phases and losses for the two types of circularly polarized light, thereby generating non-reciprocal transmission of the light field in both forward and reverse directions. The polarization field generated in the medium due to the presence of the light field can be expressed as formula (1):

[0069] (1);

[0070] in, Represents the polarization field in Components in direction, The electric field corresponding to the applied light location is Components in direction, The vacuum permittivity, Let be the polarizability matrix of the medium. When At that time, the medium is suitable for right-handed circularly polarized light ( Circularly polarized light) and left-handed circularly polarized light ( different responses, thus non-reciprocal effects are generated. In the medium without polarity and when the circularly polarized light field polarizes the medium, the medium transitions from 0 to non-0 value, which is the principle of self-induced non-reciprocity.

[0071] Figure 2 A simple schematic diagram of an optical isolator according to an embodiment of the present disclosure is schematically shown.

[0072] According to some embodiments of the present disclosure, as Figure 2 shown, the polarization control assembly 2 includes a polarizer 21, a quarter wave plate 22, and a half wave plate 23, the polarizer 21 is adapted to change the initial laser beam into linearly polarized light; the quarter wave plate 22 is adapted to change the polarization state of the linearly polarized light, and output first polarized light; the half wave plate 23 is adapted to change the polarization state of the first polarized light, and output circularly polarized light; wherein the angles of the polarizer 21, the first wave plate 22, and the second wave plate 23 are rotated to improve the purity of the circularly polarized light.

[0073] According to some embodiments of the present disclosure, the initial laser beam is changed into linearly polarized light by the polarizer 21; the polarization state of the linearly polarized light is changed by the quarter wave plate 22, and the first polarized light is output; the polarization state of the first polarized light is changed by the half wave plate 23, and the circularly polarized light is output, realizing the conversion of the initial laser beam into circularly polarized light. Moreover, by rotating the angles of the polarizer 21, the first wave plate 22, and the second wave plate 23, the initial laser beam emitted by the laser 1 can be changed into circularly polarized light with high purity after passing through the polarization control assembly 2, improving the purity of the circularly polarized light; at the same time, the polarization effect of the circularly polarized light on the atomic ensemble can be improved, the isolation ratio of the optical isolator is increased, and the forward transmittance of the initial laser beam is improved.

[0074] According to some embodiments of the present disclosure, the initial laser beam polarized into circularly polarized light with high purity by passing through the above optical isolator can realize the function of a circularly polarized light polarizer with high purity, and an additional wave plate can be additionally added to change the circularly polarized light back to linearly polarized light with the same polarization direction as the initial laser beam.

[0075] According to some embodiments of the present disclosure, by Figure 2As can be seen, the optical isolator comprises a housing, which is an aluminum cuboid housing with a size of 16 mm x 16 mm x 40 mm, and inside the housing, there is a polarizer 21, a 795 nm quarter-wave plate 22, a 795 nm half-wave plate 23, and a cubic atomic cell 3 with a side length of 10 mm. The three circular polarizing optical devices (i.e., the polarization control assembly 2) with a diameter of 12.7 mm, i.e., the polarizer 21, the quarter-wave plate 22, and the half-wave plate 23, are all installed in an aluminum barrel-shaped housing that cooperates with the cuboid housing to rotate the angles of the polarizer 21, the quarter-wave plate 22, and the half-wave plate 23 and simultaneously lock the angles of the polarizer 21, the quarter-wave plate 22, and the half-wave plate 23.

[0076] According to some embodiments of the present disclosure, the angle adjustment mode of the three polarizing optical devices is as follows: first, install the polarization control assembly 2 into the cuboid housing and test without placing the atomic cell 3. Place the above optical isolator at the exit position of the 795 nm laser 1 so that the initial laser beam passes through the polarizer 21, the quarter-wave plate 22, and the half-wave plate 23 in turn. Place a power meter behind the above optical isolator, rotate the angle of the polarizer 21 so that the power of the laser beam emitted by the above optical isolator reaches a maximum value, at which time the angle of the polarizer 21 coincides with the polarization of the laser 1, and the angle of the polarizer 21 is fixed and locked. Next, place a polarization analyzer behind the above optical isolator, which requires setting an optical intensity attenuator between the polarization analyzer and the optical isolator to attenuate the laser power, and then rotate the angles of the quarter-wave plate 22 and the half-wave plate 23 so that the laser polarization measured by the polarization analyzer is as pure circularly polarized light (either right-handed circularly polarized light or left-handed circularly polarized light) as possible, at which time the angles of the quarter-wave plate 22 and the half-wave plate 23 are locked.

[0077] According to some embodiments of the present disclosure, the atomic ensemble comprises atoms having a specific transition, wherein the specific transition is a transition from a lower energy level with angular momentum L to an upper energy level with angular momentum L-1 or L.

[0078] Figure 3 A schematic diagram of the D1 transition energy level of rubidium 87 atoms according to an embodiment of the present disclosure is schematically shown.

[0079] According to some embodiments of the present disclosure, when we select a specific atomic transition energy level, for example, the D1 transition of rubidium 87 atoms, as shown in Figure 3 , after polarizing the rubidium 87 atomic gas with right-handed circularly polarized light (circularly polarized light) , the ground state energy level of the rubidium 87 atoms is polarized to Due to the selection transition rule, the rubidium 87 atom will no longer absorb right circularly polarized light but can absorb left circularly polarized light (RCP) On this basis, the polarization control component 2 is used to ensure that the initial laser beam generated by the laser 1 passing through in the positive direction is right circularly polarized light at the position of the atomic ensemble. The atomic ensemble polarized by the right circularly polarized light will hardly absorb the right circularly polarized light. At this time, the reverse left circularly polarized light will experience strong absorption of the atomic ensemble, and the reverse right circularly polarized light cannot pass through the polarization control component 2.

[0080] According to some embodiments of the present disclosure, the self-induced non-reciprocity phenomenon in the D1 transition of the rubidium 87 atom is that the absorption of the left circularly polarized light by the rubidium 87 atom is much larger than that of the right circularly polarized light after the rubidium 87 atom is polarized by the right circularly polarized light. The D1 transition of the rubidium 87 atom can be used to make an optical isolator; in some other cases, for example, the D2 transition of the rubidium 87 atom, this phenomenon is that the absorption of the right circularly polarized light by the medium polarized by the right circularly polarized light is much larger than that of the left circularly polarized light. At this time, it is impossible to realize the optical isolator, but this phenomenon still belongs to the self-induced non-reciprocity phenomenon.

[0081] According to some embodiments of the present disclosure, the above-mentioned optical isolator needs the presence of the forward light (i.e. the initial laser beam generated by the laser 1) to realize the isolation of the reverse light (i.e. the reverse circularly polarized light) and prevent the reverse circularly polarized light from being incident on the laser 1, and the light intensity of the forward light needs to be more than one order of magnitude higher than that of the reverse light.

[0082] According to some embodiments of the present disclosure, the atomic ensemble further comprises a buffer gas, and the buffer gas is suitable for increasing the absorption linewidth of the atomic ensemble to the reverse left circularly polarized light, so as to increase the working bandwidth of the optical isolator.

[0083] According to some embodiments of the present disclosure, by arranging the buffer gas in the atomic ensemble, the absorption linewidth of the atomic ensemble to the reverse left circularly polarized light can be increased, and the working bandwidth of the optical isolator is increased.

[0084] According to some embodiments of the present disclosure, the absorption linewidth refers to the frequency range in which the atomic ensemble can effectively absorb photons. The wider the range of the absorption linewidth, the wider the frequency band in which the atomic ensemble can produce significant absorption to the laser. The working bandwidth refers to the frequency range in which the optical isolator can effectively isolate the reverse light. The frequency range is usually limited by the absorption linewidth of the atomic ensemble.

[0085] In an illustrative embodiment, the atomic gas chamber 3 is filled with rubidium 87 atom gas and 200 Torr of nitrogen gas as a buffer gas.

[0086] According to some embodiments of the present disclosure, the outside of the atomic cell 3 is provided with a heating belt and a sheet-shaped permalloy, the heating belt is suitable for increasing the temperature of the atomic ensemble to increase the density of the atomic ensemble, thereby increasing the absorption of the atomic ensemble to the reverse circularly polarized light; the sheet-shaped permalloy is suitable for shielding external magnetic fields to prevent the magnetic fields from destroying the polarization process of the atomic ensemble and the circularly polarized light.

[0087] According to some embodiments of the present disclosure, the heating belt and the sheet-shaped permalloy are both wrapped outside the atomic cell 3, wherein the heating belt is used to increase the density of rubidium atoms in the cell by increasing the temperature to increase the absorption of the atoms to the reverse light, thereby increasing the isolation ratio of the optical isolator; the sheet-shaped permalloy is used to shield external magnetic fields, prevent all magnetic fields including the magnetic field perpendicular to the direction of light propagation from destroying the polarization process of the atomic ensemble and the circularly polarized light, thereby destroying the isolation effect of the optical isolator.

[0088] Figure 4 The polarization transformation schematic diagram of the optical isolator according to the embodiments of the present disclosure is schematically shown.

[0089] According to some embodiments of the present disclosure, as shown in Figure 4 , the reverse circularly polarized light includes reverse left-handed circularly polarized light and reverse right-handed circularly polarized light, the reverse left-handed circularly polarized light is absorbed by the atomic ensemble, and the reverse right-handed circularly polarized light is isolated by the polarizer 21 in the polarization control assembly 2; by Figure 4 It can be seen that the reverse right-handed circularly polarized light is changed into reverse linearly polarized light by the quarter-wave plate 22, and the reverse linearly polarized light cannot pass through the polarizer 21, thereby realizing the isolation of the reverse right-handed circularly polarized light and preventing the reverse circularly polarized light from being incident on the laser 1.

[0090] Figure 5 The isolation effect test optical path diagram of the optical isolator according to the embodiments of the present disclosure is schematically shown.

[0091] According to some embodiments of the present disclosure, as shown in Figure 5As shown, a beam splitter 10 is placed between the laser 1 and the optical isolator described above, and the transmittance of the reverse laser (i.e. the reverse circularly polarized light) can be measured on the photodetector 11 (Thorlabs APD410A). The photodetector 11 is suitable for converting the detected optical signal of the finally transmitted reverse laser into an electrical signal. The reverse laser and the forward initial laser beam are both derived from the same laser 1. By setting the acousto-optic modulator 8 (AOM) to shift the frequency of the reverse laser by 80 MHz, interference caused by the weak signal measurement is avoided. In addition, in order to further improve the signal-to-noise ratio of the weak signal measurement, the chopper 9 (Chopper, Thorlabs MC2000) is used to modulate the light intensity of the reverse laser, and the lock-in amplifier (Zurich MFLI 500 kHz) is used to demodulate the electrical signal obtained by the photodetector 11 to obtain the transmission signal of the reverse laser. The transmission signal is further processed to obtain the transmittance of the reverse laser.

[0092] According to some embodiments of the present disclosure, the polarization of the forward light (i.e. the initial laser beam) is horizontal polarization (H), and the transmittance of the two circularly polarized lights of the reverse laser (i.e. the reverse left-handed circularly polarized light and the reverse right-handed circularly polarized light) is detected respectively. Figure 5 The test optical path shown simulates the reflection of the laser emitted by the laser 1 in the subsequent optical path in the real use scenario, and measures the transmittance of the reflection signal through the optical isolator described above to characterize the properties of the optical isolator. Here, the 80MHz frequency shift of the reflected light (i.e. the reverse laser) is much smaller than the isolation bandwidth of the optical isolator, and does not affect the measurement results in the real scenario. In a normal use scenario, the optical isolator described above only needs to be placed in front of the laser 1, and the angles of the polarizing plate 21, the quarter 22 and half 23 wave plates can be adjusted to achieve the isolation effect. Other optical devices such as acousto-optic modulator 8, chopper 9, beam splitter 10 and photodetector 11 in the test optical path are not required.

[0093] Figure 6 The reverse light transmittance measurement result diagram of the optical isolator according to an embodiment of the present disclosure is schematically shown.

[0094] According to some embodiments of the present disclosure, under the condition that the atomic cell temperature is 114℃ and the forward 795nm laser power is 46mW, the transmittance of two circularly polarized lights of 2.5mW reverse laser is tested respectively. As shown in Figure 6 As shown, the horizontal axis represents the laser frequency detuning, and the unit is GHz. The vertical axis represents the transmittance. As shown in Figure 6 As can be seen, the transmittance of the left-handed circularly polarized light is close to 10 -4The transmittance is higher at the far detuned position; however, right-handed circularly polarized light exhibits low transmittance across the entire test frequency range. This is because the low transmittance of left-handed polarized light is due to absorption by the atomic ensemble, while the low transmittance of right-handed polarized light is due to the blocking effect of the polarizer, and is almost independent of frequency. The aforementioned optical isolator can achieve 34 dB isolation for randomly polarized reverse light, and can achieve an isolation effect of over 30 dB in the 3.9 GHz frequency range. The aforementioned optical isolator has a high loss for forward light, reaching 6.5 dB, but this loss mainly originates from the uncoated gas chamber glass wall. Optimizing the uncoated gas chamber glass wall can reduce the insertion loss to about 1 dB.

[0095] Figure 7 The schematic diagram illustrates the working principle of an optical isolator according to another embodiment of the present disclosure.

[0096] According to a second aspect of this disclosure, an optical isolator is provided, such as Figure 7 As shown, the optical isolator includes a single-mode waveguide 4 and an optical medium 5. The single-mode waveguide 4 is suitable for transmitting the initial laser beam generated by the laser 1, and an evanescent field is formed on the surface of the single-mode waveguide 4. The optical medium 5 is disposed adjacent to the single-mode waveguide 4 so that the optical medium 5 is polarized by the evanescent field. Part of the initial laser beam is reflected to form a first laser beam. The first laser beam is transmitted in the opposite direction along the single-mode waveguide 4, and a reverse evanescent field is formed on the surface of the single-mode waveguide 4. The main circularly polarized light component of the reverse evanescent field is absorbed by the optical medium 5 to isolate the first laser beam and prevent the first laser beam from incident on the laser 1.

[0097] According to some embodiments of this disclosure, an initial laser beam generated by a laser 1 is transmitted using a single-mode waveguide 4, and an evanescent field is formed on the surface of the single-mode waveguide 4. An optical medium 5 is placed adjacent to the single-mode waveguide 4, causing the optical medium 5 to be polarized by the evanescent field. Part of the initial laser beam is reflected to form a first laser beam. The first laser beam propagates in the opposite direction along the single-mode waveguide 4, forming a reverse evanescent field on the surface of the single-mode waveguide 4. The main circularly polarized light component of the reverse evanescent field is absorbed by the optical medium 5, thereby isolating the first laser beam and preventing it from incident on the laser 1. This achieves the isolation of forward light (i.e., the initial laser beam) from reverse light (i.e., the first laser beam) in the field of integrated optics, without a magnetic field or any external driving force. The optical isolator of this disclosure does not require a magnetic optical isolator during the absorption or isolation of the main circularly polarized light component of the reverse evanescent field; therefore, it is also called a non-magnetic optical isolator.

[0098] According to some embodiments of the present disclosure, when the initial laser beam is transmitted inside the single-mode waveguide 4, due to the difference in refractive index between the single-mode waveguide 4 and the surrounding medium, the initial laser beam not only propagates inside the single-mode waveguide 4, but also forms a very thin light field (also known as evanescent field) near the surface of the single-mode waveguide 4. The evanescent field is weak in intensity, but it can still interact with the external environment, for example, the evanescent field interacts with the optical medium 5.

[0099] According to some embodiments of the present disclosure, when the initial laser beam is transmitted in the single-mode waveguide 4, the component of circularly polarized light corresponding to the light field (also known as evanescent field) formed by the initial laser beam above the single-mode waveguide 4 is related to the transmission direction of the initial laser beam in the single-mode waveguide 4. When the initial laser beam is transmitted forward in the single-mode waveguide 4, the component of circularly polarized light corresponding to the evanescent field is mainly left-handed circularly polarized light. When the initial laser beam is transmitted backward in the single-mode waveguide 4, the component of circularly polarized light corresponding to the evanescent field is mainly right-handed circularly polarized light.

[0100] According to some embodiments of the present disclosure, when the initial laser beam is transmitted forward in the single-mode waveguide 4, the component of circularly polarized light corresponding to the evanescent field generated above the single-mode waveguide 4 is mainly left-handed circularly polarized light. After the evanescent field generated by the forward initial laser beam polarizes the optical medium 5, the optical medium 5 produces strong absorption to the right-handed circularly polarized light component corresponding to the backward evanescent field, so as to isolate the backward evanescent field and prevent the backward evanescent field from being incident on the laser 1.

[0101] According to some embodiments of the present disclosure, the optical medium 5 includes any one of atomic ensemble and doped solid-state system. As shown in FIG. 1, the optical medium 5 is arranged above the single-mode waveguide 4. The optical medium 5 is an atomic cell, and the atomic ensemble is arranged in the atomic cell. The atomic cell is arranged above the single-mode waveguide 4. The doped solid-state system includes a base material and a doping substance. The doping substance includes a substance doped with an element other than the base material itself, for example, the doped solid-state system is lithium niobate doped with rare earth ions. Figure 7

[0102] According to some embodiments of the present disclosure, the optical isolator further includes a protective layer 6 and a chip substrate 7. The protective layer 6 is adapted to protect the structure of the single-mode waveguide 4, so as to prevent the single-mode waveguide 4 from being physically damaged by external factors such as bending, extrusion or scratching. The chip substrate 7 is adapted to support the optical isolator including the single-mode waveguide 4 and the optical medium 5.

[0103] According to some embodiments of the present disclosure, the optical isolator is integrated on a photonic integrated chip, which realizes the isolation of forward light (i.e. initial laser beam) and backward light (i.e. first laser beam) in the field of integrated optics without magnetic field and without applying any external driving.

[0104] ​According to a third aspect of the present disclosure, there is provided a light source device comprising a laser 1 and the optical isolator described above. The laser 1 is adapted to provide an initial laser beam; and the optical isolator described above is adapted to allow the initial laser beam to pass in a forward direction and to block the portion of the initial laser beam (i.e. the first laser beam) that returns along the original path from entering the laser 1.

[0105] According to some embodiments of the present disclosure, by providing the light source device comprising the laser 1 and the optical isolator described above, it is possible to ensure that the initial laser beam generated by the laser 1 passes through while preventing the reflected laser from entering the laser 1, thereby reducing the impact on the mode and frequency stability of the laser generated by the laser 1.

[0106] In an illustrative embodiment, the light source device comprises the laser 1, the polarization control assembly 2 and the atomic gas cell 3, and the polarization control assembly 2 and the atomic gas cell 3 together constitute the optical isolator.

[0107] In an illustrative embodiment, the light source device comprises the laser 1, the single-mode waveguide 4 and the optical medium 5, and the single-mode waveguide 4 and the optical medium 5 constitute the optical isolator integrated on a photonic integrated chip.

[0108] According to some embodiments of the present disclosure, the light source device described above further comprises a polarization analyzer adapted to monitor the polarization state of the initial laser beam output by the optical isolator to adjust the spatial position of the optical isolator in real time, thereby improving the purity of the laser beam output by the optical isolator.

[0109] According to some embodiments of the present disclosure, by providing the polarization analyzer, it is possible to monitor the polarization state of the initial laser beam output by the optical isolator and adjust the spatial position of the optical isolator in real time, and adjust the angles of the polarizer 21, the quarter-wave plate 22 and the half-wave plate 23 in the polarization control assembly 2, thereby improving the purity of the laser beam output by the optical isolator.

[0110] It will be appreciated by persons skilled in the art that features recited in the various embodiments and / or claims of the present disclosure can be combined or / and integrated in a variety of ways, even if such combinations or integrations are not expressly recited in the present disclosure. In particular, features recited in the various embodiments and / or claims of the present disclosure can be combined and / or integrated in a variety of ways without departing from the spirit and teachings of the present disclosure. All such combinations and / or integrations are within the scope of the present disclosure.

[0111] The above describes embodiments of the present disclosure. However, these embodiments are merely for illustrative purposes, and are not intended to limit the scope of the present disclosure. Although each embodiment is described above separately, this does not mean that the measures in each embodiment cannot be used advantageously in combination. The scope of the present disclosure is defined by the appended claims and their equivalents. Those skilled in the art can make various substitutions and modifications without departing from the scope of the present disclosure, and these substitutions and modifications should all fall within the scope of the present disclosure.

Claims

1. An optical isolator, comprising, Comprising: a polarization control component adapted to change a polarization state of an initial laser beam generated by a laser, outputting circularly polarized light, the circularly polarized light comprising right-handed circularly polarized light and left-handed circularly polarized light; and an atomic cell, the atomic cell internally provided with an atomic ensemble, the atomic ensemble being polarized by the circularly polarized light; wherein part of the circularly polarized light outputted by the atomic ensemble will return along the original path, forming counter circularly polarized light, the counter circularly polarized light comprising counter left-handed circularly polarized light and counter right-handed circularly polarized light; the counter left-handed circularly polarized light being absorbed by the atomic ensemble, and the counter right-handed circularly polarized light being isolated by the polarization control component to achieve isolation of the counter circularly polarized light and prevent the counter circularly polarized light from being incident on the laser; wherein the polarization control component comprises: a polarizer adapted to change the initial laser beam into linearly polarized light; a quarter-wave plate adapted to change a polarization state of the linearly polarized light, outputting first polarized light; and a half-wave plate adapted to change a polarization state of the first polarized light, outputting the circularly polarized light; wherein the purity of the circularly polarized light is improved by rotating the angles of the polarizer, the quarter-wave plate and the half-wave plate.

2. The optical isolator of claim 1, wherein, The atomic ensemble and the circularly polarized light have a self-induced non-reciprocal phenomenon, which is that after the atomic ensemble is polarized by the circularly polarized light, the circularly polarized light and the counter circularly polarized light will produce different phases and losses when transmitting in opposite directions in the atomic ensemble, to achieve non-reciprocal transmission of the circularly polarized light and the counter circularly polarized light.

3. The optical isolator of claim 1 or 2, wherein, The atomic ensemble comprises atoms with specific transitions, wherein the specific transitions are transitions from a lower energy level with angular momentum L to an upper energy level with angular momentum L-1 or L.

4. The optical isolator of claim 3, wherein, The atomic ensemble further comprises a buffer gas, which is adapted to increase the absorption linewidth of the atomic ensemble to the counter left-handed circularly polarized light, to improve the working bandwidth of the optical isolator.

5. The optical isolator of claim 4, wherein, The atomic cell is externally provided with a heating belt and a sheet-shaped permalloy, the heating belt is adapted to increase the temperature of the atomic ensemble, to increase the density of the atomic ensemble, and thus to increase the absorption of the atomic ensemble to the counter circularly polarized light; the sheet-shaped permalloy is adapted to shield external magnetic fields, to prevent the magnetic fields from destroying the polarization process of the atomic ensemble and the circularly polarized light.

6. A light source apparatus, wherein, Comprising: a laser adapted to provide an initial laser beam; and the optical isolator of any one of claims 1-5, adapted to allow the initial laser beam to pass forward and prevent part of the initial laser beam returning along the original path from being incident on the laser.

7. The light source apparatus according to claim 6, wherein Further comprising: a polarization analyzer adapted to monitor the polarization state of the initial laser beam outputted by the optical isolator, to adjust the spatial position of the optical isolator in real time, and thus to improve the purity of the laser beam outputted by the optical isolator.

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