Laser system for reducing relative intensity noise

Abstract

The present disclosure relates to a laser system for reducing the relative intensity noise (RIN) of an optical signal, the laser system comprising: at least one pump laser configured to provide a pump signal having a given RIN level; a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal; and a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved, wherein a RIN level of the frequency-converted optical signal is lower than the RIN level of the pump signal at least in a range of frequencies, optionally wherein first-order contributions to the RIN of the frequency-converted optical signal are suppressed or reduced. The present disclosure further relates to a method of reducing the relative intensity noise (RIN) of an optical signal. The disclosure further relates to a quantum computing system.

Description

[0001] LASER SYSTEM FOR REDUCING RELATIVE INTENSITY NOISE

[0002] Technical field

[0003] The present disclosure relates to a laser system. The present disclosure further relates to a laser system for reducing the relative intensity noise of an optical signal, such as a frequency-converted optical signal, and a method of achieving the same. The disclosure further relates to a quantum computing system.

[0004] Background

[0005] Nonlinear optical processes, such as second-harmonic generation (SHG), are widely used in advanced optical systems to convert light from one frequency to another. These processes are critical in applications such as telecommunications, quantum computing, precision spectroscopy, and metrology. The efficiency and reliability of these processes depend on maintaining stable and low-noise optical outputs, particularly in systems requiring high precision and sensitivity.

[0006] One significant challenge in such systems is relative intensity noise (RIN), which may refer to fluctuations in the intensity of the laser or optical signal relative to its average intensity. RIN can arise from various factors, including intrinsic noise in the laser source, thermal fluctuations, and environmental disturbances. These fluctuations can propagate and even amplify during nonlinear conversion, degrading system performance and limiting the utility of the optical output in noise-sensitive applications.

[0007] As demand grows for compact, efficient, and low-noise optical systems, there remains a need for innovative systems and methods to address RIN in nonlinear optical conversion processes.

[0008] Summary

[0009] The above-mentioned challenges are solved by providing a laser system for reducing the relative intensity noise (RIN) of an optical signal, the laser system comprising: at least one pump laser configured to provide a pump signal; a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal; and a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved. A technicaleffect hereof is that at least first-order contributions to the RIN of the frequency-converted optical signal are suppressed or reduced.

[0010] In optical systems employing optical frequency conversion, it is typically of interest to maximize the conversion efficiency of the frequency conversion process, such that the output power from said process is maximized. Generally, many nonlinear frequency conversion processes, such as frequency doubling, as well as sum and difference frequency generation, require phase matching to be efficient. Phase matching generally refers to matching the phase of the interacting optical signals in a frequency conversion element for carrying out the frequency conversion process. An optimal phase matching typically implies that a constant phase relationship is maintained over the interaction length of the conversion element. In a perfectly phase-matched condition, there is a zero phase mismatch. More generally, a phase mismatch of zero, or close to zero, is often desired in order to obtain an effective nonlinear interaction in the frequency conversion element. This will cause the conversion efficiency of the process to be maximized for a given input pump power. In other words, a phase mismatch of zero generally results in a maximized output power of the frequency-converted optical signal, which is typically desired.

[0011] However, the present inventors have realized that a number of benefits can be achieved by operating the frequency conversion element away from the optimal phasematching condition, such that a non-zero phase mismatch is achieved. Such a condition is typically considered non-optimal, since it implies a non-maximized conversion efficiency. Most importantly, and surprisingly, the present inventors have realized that the relative intensity noise (RIN) of the frequency-converted optical signal can be suppressed or reduced, at least for a range of noise frequencies, as a consequence hereof, as further described herein. Specifically, the laser system disclosed herein may comprise a tuning mechanism configured to adjust the phase-match away from the optimal phase-matching condition, such that a non-zero phase mismatch is achieved. In other words, the tuning mechanism may be configured to adjust the phase-match to an operating point of nonmaximum conversion efficiency. As an example, the tuning mechanism may comprise one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element. Generally, the phase mismatch, and thereby the conversion efficiency, depends on the temperature setpoint. Thus, by varying, or adjusting, the temperature of the frequency conversion element away from what is typically considered an optimal temperature setpoint, a non-zero phase mismatch can be achieved. Interestingly, such a temperature detuning causes a RIN reduction for a given nonlinearity and input power. As a slight drawback hereof, the conversion efficiency is moved away from the optimal conversion efficiency corresponding to the maximum output power. However, the present inventors havefound, through experiments and computer simulations, regions and operating points, wherein a significant RIN reduction is achieved without too much sacrifice on the conversion efficiency. In some cases, a RIN reduction exceeding 20 dB is demonstrated. This RIN reduction may be understood relative to the pump signal, i.e., how much the RIN of the optical signal is reduced compared to the RIN of the pump signal.

[0012] In accordance with some embodiments, the disclosed laser system comprises at least one pump laser, such as a continuous-wave laser, configured to provide a pump signal; a frequency conversion element, such as a nonlinear crystal or nonlinear waveguide, configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal; and one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element, wherein the temperature controller(s) are configured to adjust the temperature setpoint such that a non-zero phase mismatch is achieved.

[0013] In accordance with some embodiments, the disclosed laser system comprises at least one pump laser, wherein said pump laser is a continuous-wave laser configured to provide a continuous-wave pump signal; a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal, wherein the frequency conversion element is a periodically poled nonlinear waveguide or a periodically poled nonlinear crystal; and a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved, wherein the non-zero phase mismatch is achieved by a temperature detuning of the frequency conversion element.

[0014] In accordance with some embodiments, the disclosed laser system comprises two or more pump lasers, wherein the two or more pump lasers comprise a first pump laser for providing a first pump signal at a first pump wavelength, and a second pump laser for providing a second pump signal at a second pump wavelength; a frequency conversion element configured to frequency convert the pump signals by sum-frequency generation (SFG) of the first and second pump signals, whereby a sum frequency-converted optical signal is generated, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signals and the frequency-converted optical signal; and a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch isachieved, optionally wherein the non-zero phase mismatch is achieved by a temperature detuning of the frequency conversion element or wherein the tuning mechanism is configured to apply an external electric field across the frequency conversion element.

[0015] In accordance with some embodiments, the disclosed laser system comprises at least one pump laser configured to provide a pump signal; a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal, wherein the frequency conversion element is a nonlinear crystal arranged at an angle with respect to a laser beam comprising the pump signal; and a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved, wherein the tuning mechanism is configured to adjust the angle of the nonlinear crystal to achieve the non-zero phase mismatch or wherein the tuning mechanism is configured to apply an external electric field across the nonlinear crystal to achieve the non-zero phase mismatch.

[0016] In accordance with some embodiments, the disclosed laser system comprises at least one pump laser configured to provide a pump signal having a given RIN level; a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal; and a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved, wherein a RIN level of the frequency-converted optical signal is lower than the RIN level of the pump signal at least in a range of frequencies.

[0017] In accordance with some embodiments, the disclosed laser system comprises at least one pump laser configured to provide a pump signal having a first RIN level; a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal having a second RIN level, wherein the frequency conversion element has a conversion efficiency that depends on a phase-matching condition; and a tuning mechanism configured to adjust the phase-match to an operating point of non-maximum conversion efficiency, such that a non-zero phase mismatch is achieved; wherein the second RIN level is lower than the first RIN level at least in a range of frequencies, such as in a range of frequencies above 1 kHz. In preferred embodiments, the second RIN level is at least 10 dB lower, such as at least 20 dB lower, than the first RIN levelin a range of frequencies above 1 kHz. 6. As an example, the second RIN level is lower than the first RIN level at least in a range of frequencies from about 103Hz to about 10® Hz.

[0018] A further aspect provides a method of reducing the relative intensity noise (RIN) of an optical signal, the method comprising the steps of: providing a pump signal having a first RIN level; directing the pump signal through a frequency conversion element to generate a frequency-converted optical signal having a second RIN level; and adjusting a phasematching condition of the frequency conversion element to an operating point of nonmaximum conversion efficiency, such that a non-zero phase mismatch is achieved; wherein the adjustment step causes the second RIN level to be lower than the first RIN level at least in a range of frequencies.

[0019] A further aspect provides a quantum computing system. The quantum computing system may comprise a plurality of qubits and the laser system as described herein. In such a system, the laser system is optically coupled to the qubits. The laser system is configured to deliver the frequency-converted optical signal to one or more of the qubits. The frequency-converted optical signal may be configured for one or more purposes selected from the group of: cooling of atoms or ions, trapping of atoms or ions, qubit initialization, manipulation of qubit states, and / or combinations thereof. Specifically, the frequency-converted optical signal may be configured to control and / or read out a quantum state of the qubits. Control operations may include laser cooling and / or trapping of qubits, initializing them into a specific state, and / or manipulating their quantum states to perform logic gates. The laser system may be suitable for coherent state control of a quantum system.

[0020] In some embodiments, the plurality of qubits are neutral atoms. As an example, the neutral atoms may be selected from the group of: Ytterbium (Yb), Rubidium (Rb), Barium (Ba), or Strontium (Sr). The frequency-converted optical signal may be configured for laser cooling or trapping of neutral atoms. More specifically, the frequency-converted optical signal may be configured for magneto optical trapping (MOT) of the neutral atoms. The quantum computing system may comprise an atom trap for storing qubits in stable quantum states. The laser system is preferably configured to deliver the frequency-converted optical signal to the atom trap to interact with the qubits.

[0021] In some embodiments, the plurality of qubits are trapped ions. As an example, the trapped ions may be selected from the group of: Ytterbium ions (Yb+), Barium ions (Ba+), or Rubidium ions (Rb+). The frequency-converted optical signal may be configured for laser cooling or trapping of ions. Additionally, or alternatively, the frequency-converted optical signal may be configured for qubit initialization, state manipulation of quantum logic gates, and / or combinations thereof. The quantum computing system may comprise an ion trap forstoring qubits in stable quantum states. The laser system is preferably configured to deliver the frequency-converted optical signal to the ion trap to interact with the qubits.

[0022] The presently disclosed system and method is particularly advantageous for applications, where a low RIN in the frequency-converted optical signal is desired, at the expense of some decrease in the conversion efficiency. Examples of such applications include quantum computing and quantum communication, atomic clocks, high-resolution spectroscopy, interferometric sensing and metrology, quantum optics, biophotonics and medical imaging, such as optical coherence tomography (OCT) and multiphoton microscopy. The disclosed system and method could be relevant to other applications not explicitly listed here.

[0023] Brief description of the drawings

[0024] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0025] Fig. 1 shows an embodiment of a laser system according to the present disclosure, wherein the laser system comprises a pump laser, a frequency conversion element, and a tuning mechanism.

[0026] Fig. 2 shows an embodiment of a laser system according to the present disclosure, wherein the laser system comprises a pump laser, a frequency conversion element, a tuning mechanism, and one or more optical amplifiers.

[0027] Fig. 3 shows an embodiment of a laser system according to the present disclosure, wherein the laser system comprises a continuous-wave laser, one or more fiber amplifiers, a periodically poled nonlinear waveguide, and one or more temperature controllers.

[0028] Fig. 4 shows the converted power, P2a), versus the nonlinearity, y P^z, for three different input powers: Po- 8P, Po, and Po+ 8P, wherein y is the nonlinear coefficient, Pothe average pump power, 8P the fluctuations in power around Po, and z the propagation distance inside the frequency conversion element.

[0029] Fig. 5 shows the converted power versus the nonlinearity, in a scenario wherein there is a non-zero phase mismatch of the pump signal and the converted signal.

[0030] Fig. 6 shows the converted power versus the nonlinearity, in a scenario wherein there is a non-zero phase mismatch of the pump signal and the converted signal, and further wherein the frequency conversion element is operated in the pump depletion regime.Fig. 7 shows simulated data of the power versus time for four different scenarios, wherein the power is plotted relative to the input power.

[0031] Fig. 8 shows the conversion efficiency versus the phase mismatch and nonlinearity, wherein the two independent parameters are provided as unitless numbers.

[0032] Fig. 9 shows the relative intensity noise (RIN) contribution in dB (labelled ‘RIN penalty’) versus the phase mismatch and nonlinearity.

[0033] Fig. 10 shows the conversion efficiency and the RIN contribution (labelled ‘RIN penalty’) in dB versus the phase mismatch for a given nonlinearity.

[0034] Fig. 11 shows the conversion efficiency and the RIN contribution (labelled ‘RIN penalty’) in dB versus the phase mismatch for a given nonlinearity, which is higher than the example of Fig. 10.

[0035] Fig. 12 shows experimental data of relative intensity noise (RIN) in units of dBc / Hz. The horizontal axis shows the frequency of the noise in units of Hz.

[0036] Fig. 13 shows experimental data of the output power (mW) of a frequency-converted optical signal versus the temperature setpoint (°C) of the frequency conversion element, for a constant pump power.

[0037] Fig. 14 shows a graph of the relative intensity noise (RIN) contribution in dB, here labelled as the ‘RIN penalty’, versus the conversion efficiency in percentage, for a constant pump power.

[0038] Fig. 15 shows the relative intensity noise (RIN) in dB versus the conversion efficiency in percentage for a range of noise frequencies in Hz, for a constant pump power.

[0039] Fig. 16 shows the RIN contribution in dB from a second-harmonic generation frequency-conversion process for a range of noise frequencies in Hz.

[0040] Fig. 17 shows the output power (normalized) from a frequency conversion process versus the temperature setpoint (in °C) of the frequency conversion element, for a constant pump power.

[0041] Fig. 18 shows an embodiment of a quantum computing system according to the present disclosure, said quantum computing system comprising the laser system disclosed herein.

[0042] Fig. 19 shows an embodiment of a laser system according to the present disclosure, wherein the laser system is configured to output a low-RIN optical signal at about 1 pm.Detailed description

[0043] A better understanding of the features and advantages of the present disclosure will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of embodiments of the present disclosure are utilized, and the accompanying drawings.

[0044] Relative intensity noise

[0045] Relative intensity noise (RIN) may be understood as a measure of the fluctuations in the optical power of a laser or light source relative to its average power. RIN is typically expressed as the ratio of the power spectral density of intensity noise to the square of the average intensity, often in units of dB / Hz. It may also be expressed in units of dBc / Hz, where dBc means dB relative to the carrier, such that dBc / Hz means dBc in a 1-Hz bandwidth. RIN typically quantifies noise across different frequencies and is often considered critical in evaluating the stability and performance of optical systems, such as laser systems. In the present context, the term ‘RIN level’ may be understood as the magnitude of the relative intensity noise at a specific frequency or over a range of frequencies. Thus, the term ‘RIN level’ may be understood to encompass the terms ‘RIN curve’ or ‘RIN characteristic’, where the relative intensity noise (RIN) is plotted against frequency.

[0046] Phase matching

[0047] Phase matching may be understood as a condition where interacting optical signals in a frequency conversion element, such as the input pump signal and the frequency-converted optical signal, maintain a constant phase relationship over the interaction length of the conversion element. This may ensure constructive interference, maximizing the efficiency of energy transfer and enhancing the nonlinear conversion process, such as in second-harmonic generation (SHG) or sum-frequency generation (SFG). An optimal phase-matching condition may be understood as a condition in which the conversion efficiency is maximized, typically corresponding to a phase mismatch of zero. The present inventors have realized that a number of advantages can be achieved by operating the conversion element away from the optimal phase-matching condition, i.e., by ensuring a non-zero phase mismatch. In particular, a large reduction in the RIN of the frequency-converted optical signal can be achieved, as further explained herein. Specifically, the inventors have realized that the RIN of the frequency-converted optical signal is lower than the RIN of the signal, which is input to the frequency conversion element. Thus, the RIN of the output signal from the conversion element is lower than the RIN of the input signal, at least for a large range of frequencies, for the conditions mentioned herein. Similarly, for a given pump power, the RIN of the frequencyconverted optical signal is lower for a non-phase matched condition, compared to an optimal phase matched condition.

[0048] Second-harmonic generation

[0049] Second-harmonic generation (SHG) may be understood as a nonlinear optical process in which two photons of the same frequency interact with a nonlinear medium to generate a new photon with twice the frequency of the original photons. SHG is also referred to as frequency doubling in the technical field of optical systems and lasers. SHG is commonly used for frequency conversion in lasers, enabling the generation of new wavelengths for applications in spectroscopy, microscopy, telecommunications, and quantum optics. In preferred embodiments, the disclosed laser system comprises a frequency conversion element that is configured to frequency convert a pump signal through second-harmonic generation (SHG).

[0050] The SHG conversion process in the frequency conversion element may be described by the following set of coupled differential equations:

[0051]

[0052] where A^ is the amplitude of the pump field, such that |2&J|2is the optical power, A2a)is the amplitude of the converted field, is the imaginary unit, y is the nonlinear coefficient, A is the complex conjugate of the pump field, Ak is the phase mismatch, and z is a propagation distance, or position, measured along the length of the frequency conversion element. The phase mismatch, Ak, may be given by the difference between the wavenumbers of the pump signal and the frequency-converted optical signal. The conversion efficiency in percentage (%) per watt of input power may be expressed as the term (yL)2. To solve the set of coupled differential equations one can assume that the amplitude of the converted field is zero at the beginning of the frequency conversion element, corresponding to z = 0 such that A2a)(0) = 0, and further that the amplitude of the pump field is yfp^ at z = 0 such that / lw(0) = fp^, wherein Pois the average pump power, i.e. , the average power of the pump signal.

[0053] From a dimensional analysis, it follows that the conversion efficiency of the conversion process depends on two independent parameters, which can be expressed as the following unitless quantities: y^Poz and -^=. In other words, the conversion efficiency, rj, is a function of said parameters. The conversion efficiency is plotted against these parameters in figure 8.The first quantity, y P^z, will be referred to herein as the nonlinearity, since it expresses the strength of the nonlinearity of the frequency conversion element. As seen from the expression, the nonlinearity can be affected by three parameters: y, which is the nonlinear coefficient, which depends on the material and / or type of the frequency conversion element, Powhich is the average input power, i.e., the average power of the pump signal or the average power of an amplified pump signal, and the value of z, the propagation distance inside the frequency conversion element, which implies that a longer frequency conversion element provides a greater nonlinearity. The second quantity, -^=, is the phase mismatch relative to the term Y O, but for simplicity it may be referred to herein as the phase mismatch. The expressions for the independent parameters have been deliberately chosen such that they are unitless, which makes it possible to plot the solutions to the equations without specifying numerical values, e.g. a specific length, input power, etc. Another way to phrase this is by considering a constant nonlinear coefficient, y, and a constant input power, Po. In that case, the two quantities reduce to simply the z-coordinate along the length of the frequency conversion element, and the phase mismatch, A / c. In other words, the term Y O can be thought of as a term for ensuring that the two parameters become unitless.

[0054] In typical frequency conversion processes, the frequency conversion element, such as a nonlinear crystal, is operated in a quadratic regime, wherein the power of the frequency-converted signal scales quadratically with the input power. For a phase matched SHG process in the quadratic regime, the converted power can be expressed as P2lJJ= Y2PQL2, where L is the length of the frequency conversion element. In such cases, there is typically a 6 dB addition of noise to the relative intensity noise of the frequency-converted optical signal. It is preferred that the frequency conversion element of the present disclosure is operated outside said quadratic regime. This can be achieved by increasing the nonlinearity, e.g., through an increase of the nonlinear coefficient y, an increase of the input power Po, an increase of the length of the conversion element, and / or combinations thereof.

[0055] Advantageously, the frequency conversion element is operated in a pump depletion regime. Pump depletion may refer to a reduction in the intensity of the pump signal in a nonlinear optical process due to the transfer of its energy to generate a converted signal at a different frequency, such as in second-harmonic generation (SHG) or other frequency-mixing processes. Generally, as the pump signal propagates through the frequency conversion element, its energy is gradually converted into the desired frequency-converted optical signal, leading to a decrease in power of the pump signal. This effect becomes more significant at high conversion efficiencies, where a substantial portion of the pump energy is used in the frequency conversion process. In other words, pump depletion may beunderstood as the phenomenon that the pump power for some nonlinear process occurring in a frequency conversion element can be depleted or reduced.

[0056] Pump laser

[0057] The laser system may comprise at least one pump laser configured to provide a pump signal. In some embodiments, the pump signal has a wavelength selected in the range from about 1525 nm to about 1570 nm, such as at a wavelength of about 1530 nm or a wavelength about 1560 nm. Such wavelengths can be provided by an Erbium-based laser. In other embodiments, the pump signal has a wavelength selected in the range from about 1025 nm to about 1065 nm, such as at a wavelength of about 1026 nm or a wavelength about 1064 nm. Such wavelengths can be provided by an Ytterbium-based laser. In other embodiments, the pump signal has a wavelength selected in the range from about 1.9 pm to about 2.1 pm, such as at a wavelength of about 2 pm. Such wavelengths can be provided by a Thulium-based laser. The pump laser may be a pulsed laser for providing a pulsed pump signal or a continuous-wave laser for providing a continuous-wave pump signal. The pump laser may be configured to generate a pump signal having a predefined average power, such as a power between 1 mW and 15 W, such as between 1 mW and 10 W, such as between 10 mW and 1 W The pump laser may be a fiber laser or a solid-state laser. In some embodiments, the pump laser is a narrow-linewidth single-frequency fiber laser.

[0058] The pump signal from the pump laser generally has a given RIN level. Any optical amplifiers arranged downstream of the pump laser will typically further add intensity noise, such that the RIN level increases. The presently disclosed laser system and method have proven to reduce the RIN level, such that the RIN level of the frequency-converted optical signal is lower than the RIN level of the pump signal and / or lower than the RIN level of an amplified pump signal from one or more optional optical amplifiers. In some embodiments, the RIN of the frequency-converted optical signal is reduced by at least 10 dB relative to the RIN level of the pump laser, at least in a range of noise frequencies, as evident from the experimental data shown in figure 12.

[0059] In some embodiments, the laser system comprises two or more pump lasers. In such a case, the two or more pump lasers may comprise a first pump laser for providing a first pump signal at a first pump wavelength and a second pump laser for providing a second pump signal at a second pump wavelength. The first pump wavelength may be different from the second pump wavelength. In this case, the frequency conversion element may be configured to frequency convert the pump signal by sum-frequency generation (SFG) of the first and second pump signals. Alternatively, the frequency conversion element may beconfigured to frequency convert the pump signal through difference-frequency generation (DFG).

[0060] Amplifier

[0061] The laser system may comprise one or more optical amplifiers for amplifying an optical signal. In particular, the optical amplifier(s) may be configured to amplify the pump signal to generate an amplified pump signal. In some embodiments, the laser system comprises an optical amplifier, such as a fiber amplifier, arranged downstream of the pump laser and upstream of the frequency conversion element. In some embodiments, the optical amplifier is configured to amplify the pump signal to a power of at least 500 mW, such as at least 1 W, such as at least 2 W, such as at least 10 W, such as at least 15 W

[0062] The optical amplifier(s) may be selected from the group of optical fiber amplifiers and semiconductor optical amplifiers (SOA). In case of optical fiber amplifiers, the amplifier may be a doped-fiber amplifier, such as an erbium-doped fiber amplifier (EDFA), an ytterbium-doped fiber amplifier (YDFA), a thulium-doped fiber amplifier (TDFA), or a co-doped fiber amplifier, such as an erbium-ytterbium co-doped fiber amplifier. The optical amplifier(s) may comprise one or more pump sources, such as pump diodes, for optically pumping an active medium, such as an active fiber, in the optical amplifier. The optical amplifier(s) may be optically connected to one or more other components of the laser system, such as connected to the pump laser and / or the frequency conversion element.

[0063] In some cases, the optical amplifier(s) adds further intensity noise to the pump signal, such that the amplified pump signal has a higher relative intensity noise level compared to the pump signal from the pump laser. In other words, the optical amplifier may introduce or add intensity noise to the pump signal, such that the RIN of the amplified pump signal is increased. Generally, the RIN may depend on the power of the pump signal. In some embodiments, the pump signal from the pump laser has a first RIN level and the amplified pump signal has a second RIN level, wherein the second RIN level is higher than the first RIN level. The presently disclosed system and method have the advantage that the RIN of the frequency-converted optical signal is lower than first and / or second RIN level, at least for a range of frequencies, such as above 1 kHz. This is further explained elsewhere herein. Frequency conversion element

[0064] The laser system may comprise a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal. The frequency conversion element may be configured to frequency convert the pump signal by a nonlinear optical process, such as second-harmonic generation (SHG), sum-frequencygeneration (SFG), difference-frequency generation (DFG), other types of multiple harmonic generation, or other types of nonlinear frequency processes. The frequency conversion element is preferably configured for single-pass frequency conversion processes. This is in contrast to conversion processes that utilize an optical field enhanced by an optical cavity, i.e. , cases where a nonlinear crystal is placed inside an optical cavity or optical resonator. Alternatively, the frequency conversion element is configured for double-pass frequency conversion processes.

[0065] In some embodiments, the frequency conversion element is configured to frequency double the pump signal through SHG. In other embodiments, the laser system comprises at least two pump lasers, generating first and second pump signals, wherein the frequency conversion element is configured to frequency convert the pump signals by sum-frequency generation (SFG) of the first and second pump signals. In other embodiments, differencefrequency generation (DFG) of the pump signals is employed to generate the frequency-converted optical signal.

[0066] The frequency conversion element may be selected among the group of nonlinear crystals or nonlinear optical waveguides. In some embodiments, the frequency conversion element is a periodically poled nonlinear waveguide, such as a periodically poled lithium niobate (PPLN) waveguide. In other embodiments, the frequency conversion element is a nonlinear crystal, such as a birefringent crystal. In embodiments, wherein the conversion element is a nonlinear bulk crystal, it may be non-poled or periodically poled. The frequency conversion element may be a fiber-coupled device. Thus, the fiber-coupled device may comprise one or more optical fiber connections for connecting one or more optical fibers to the device. Generally, the laser system may comprise one or more optical fibers for optically connecting one or more of the optical components described herein, such as the pump laser and the frequency conversion element.

[0067] In some embodiments, the optical power of the frequency-converted optical signal output from the frequency conversion element is at least 50 mW, such as at least 100 mW, such as at least 200 mW. As an example, the optical power of the frequency-converted optical signal output from the frequency conversion element may be between 1 mW and 15 W, such as between 1 mW and 5 W, such as between 10 mW and 3 W, such as between 100 mW and 2 W In some embodiments, the wavelength of the frequency-converted optical signal is in a range from about 493 nm to about 614 nm, such as in a range from 493 nm to 532 nm, such as a wavelength at about 493 nm or alternatively a wavelength at about 532 nm. In other embodiments, the wavelength of the frequency-converted optical signal is in arange from about 650 nm to about 840 nm, such as in a range from 650 nm to 780 nm, such as a wavelength about 650 nm or alternatively a wavelength about 780 nm.

[0068] In some embodiments, the optical power of the pump signal is sufficiently high that the frequency conversion element is operated in a pump depletion regime. Additionally, or alternatively, the nonlinearity of the frequency conversion element is sufficiently high that the frequency conversion element is operated in the pump depletion regime. Additionally, or alternatively, the frequency conversion element may have a length sufficient to operate the frequency conversion element in the pump depletion regime. Accordingly, the frequency conversion element is preferably operated in the pump depletion regime, and there are multiple ways of achieving this as described herein above. Typically, frequency conversion elements are operated in a quadratic regime, where the frequency-converted power scales quadratically with the nonlinearity of the frequency conversion element. When operated in the quadratic regime, a phase-matched frequency conversion element typically adds intensity noise to the input signal, typically about 6 dB of noise is added. Theoretically, the intrinsic RIN contribution from the frequency conversion process cannot exceed 6 dB; however, often it is close to 6 dB.

[0069] Advantageously, the frequency conversion element, according to the present disclosure, is operated outside the quadratic regime of converted power versus nonlinearity. Even more advantageously, the frequency conversion element is operated in the pump depletion regime as explained above. An advantage hereof, is that the intrinsic RIN contribution from the frequency conversion process is further reduced. In some cases, the frequency conversion element is operated in, or near, a regime, wherein the power of the frequency-converted optical signal is substantially independent, such as entirely independent, of the power of the pump signal. The regime may correspond to a region around an operating point wherein the power of the frequency-converted optical signal is substantially independent of the power of the pump signal.

[0070] The frequency conversion element may have a conversion efficiency that depends on a phase-matching condition between the pump signal and the frequency-converted optical signal. In some embodiments, the frequency conversion element is operated in a non-phase-matched regime, such that there is a phase mismatch between the pump signal, or the amplified pump signal, and the frequency-converted optical signal. This has the technical effect that the relative intensity noise in the optical signal from the pump laser is reduced or suppressed, and / or intensity noise introduced by the optical amplifier(s) is reduced or suppressed. Advantageously, the frequency conversion element is operated both in a pump depletion regime and a non-phase-matched regime. When operated in the non-phase-matched regime, the conversion efficiency of the frequency conversion element may be lower compared to a perfectly phase-matched condition.

[0071] Tuning mechanism

[0072] The laser system may comprise a tuning mechanism. The pump signal and the frequency-converted optical signal may each have separate phases. Normally, it is desired to operate a frequency conversion element in a phase-matched condition to achieve maximum conversion efficiency, i.e. to have maximum power in the frequency-converted signal. An optimal phase-matched condition, sometimes referred to as perfect phasematching, implies that the phase mismatch is zero. However, the present inventors have realized that some benefits, such as a reduction of the RIN of the frequency-converted signal, can be achieved by operating the frequency conversion element away from the perfectly phase-matched condition. Accordingly, the tuning mechanism may be configured to adjust the phase-match away from an optimal phase-matching condition. A technical effect hereof is that a non-zero phase mismatch is achieved, whereby at least first-order contributions to the RIN of the frequency-converted optical signal are suppressed or reduced. In some cases, said contribution to the RIN may be understood as an intrinsic RIN contribution from the frequency-conversion process to the total RIN. Consequently, the intrinsic RIN, and thereby also the total RIN, of the frequency-converted optical signal is lowered, compared to the pump signal and / or compared to an amplified optical signal, which is input to the frequency conversion element. It is also lower compared to a frequency-converted optical signal of a phase-matched frequency process. There are various ways of achieving a non-zero phase mismatch, which will be described in more detail in the following.

[0073] In some embodiments, the non-zero phase mismatch is achieved by a temperature detuning of the frequency conversion element. Thus, the tuning mechanism may comprise, or constitute, one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element. In particular, the temperature controller(s) may be configured to adjust the temperature setpoint such that a non-zero phase mismatch is achieved. The temperature controller(s) may be selected from any suitable temperature controllers for controlling the temperature of the frequency conversion element. In some embodiments, the temperature controller(s) are selected from the group of thermoelectric devices, such as Peltier elements, resistive heaters, such as thin-film or wire-based heaters, miniature heating elements, liquid cooling systems combined with heating elements, and / or combinations thereof. As a specific example, the temperature controller(s) may comprise aPeltier element to adjust the temperature setpoint of the frequency conversion element. The temperature controller(s) may be thermoelectric cooler(s) (TEC).

[0074] In some embodiments, the tuning mechanism comprises two or more temperature controllers, preferably wherein the temperature controllers can be controlled individually. In some cases, the two or more temperature controllers are configured to provide a uniform temperature distribution throughout the frequency conversion element. In other cases, the two or more temperature controllers are configured to provide one or more temperature gradients, such as a single temperature gradient, throughout the frequency conversion element. Thus, the provision of two or more temperature controllers provides a more versatile tuning mechanism in that either a uniform temperature distribution or one or more temperature gradients throughout the frequency conversion element can be achieved.

[0075] In some embodiments, the non-zero phase mismatch is achieved by adjusting the phase matching condition via the electro-optic effect. In such embodiments, the tuning mechanism may comprise one or more electrodes for utilizing the electro-optic effect to achieve a non-zero phase mismatch. Alternatively, or additionally, the tuning mechanism may be configured to induce, change, or adjust, an electric field through the frequency conversion element, such as by applying a bias voltage across the frequency conversion element. The electric field may be a direct current (DC) electric field.

[0076] In some embodiments, the frequency conversion element is a nonlinear crystal, such as a birefringent crystal or a periodically poled bulk crystal. In such embodiments, the tuning mechanism may be configured to adjust the angle of the nonlinear crystal, such as the angle of a bulk crystal, to achieve a non-zero phase mismatch. Accordingly, the tuning mechanism may comprise one or more actuatable stages, a motorized platform, a rotatable platform or other suitable means for rotating and / or translating the nonlinear crystal. Consequently, the tuning mechanism may be configured to adjust the angle of the nonlinear crystal with respect to a laser beam incident on the nonlinear crystal.

[0077] Accordingly, the tuning mechanism may be selected from the group of: temperature controller(s), actuatable stage(s), electrode(s), and / or combinations thereof. Specifically, the tuning mechanism may be selected from the group of: one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element, one or more actuatable stages configured to adjust an angle of the frequency conversion element with respect to a laser beam comprising the pump signal, and / or one or more electrodes configured to apply an external electric field across the frequency conversion element.

[0078] Summarizing, the presently disclosed laser system may be configured to reduce the relative intensity noise (RIN) of a frequency-converted optical signal such as by detuning thefrequency conversion element to achieve a non-zero phase mismatch. The detuning may include a temperature detuning, i.e., a temperature adjustment, of the frequency conversion element, as further described herein above. Alternatively, in case of a nonlinear crystal, the detuning may be achieved from a rotation of the nonlinear crystal. The conversion efficiency may change in response to an adjustment of the phase mismatch. The effect of RIN reduction is even greater when the frequency conversion element is operated in a pump depletion regime. For small levels of depletion of the pump signal, a relatively low conversion efficiency, and correspondingly relatively low conversion power, is achieved if a relatively large RIN reduction is desired. In other words, the size of the RIN reduction can be adjusted through the size of the phase mismatch, but the penalty on the conversion efficiency is lower when the frequency conversion element is operated in the pump depletion regime, preferably far into the pump depletion regime.

[0079] Interface

[0080] In some embodiments, the laser system comprises an interface for controlling the desired phase mismatch, the amount of RIN reduction, the conversion efficiency, and / or combinations thereof. As an example, the interface may be provided as a graphical user interface. The laser system may comprise an electronic display for displaying the graphical user interface. The user interface may allow an end-user to select the amount of RIN reduction and / or the desired conversion efficiency by interaction with the interface.

[0081] Detailed description of the drawings

[0082] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, figures, and graphs.

[0083] Fig. 1 shows an embodiment of a laser system according to the present disclosure. In this embodiment, the laser system comprises at least one pump laser (102) configured to provide a pump signal (104). The laser system further comprises a frequency conversion element (106) configured to frequency convert the pump signal (104) to generate a frequency-converted optical signal (110). The frequency conversion element (106) has a conversion efficiency depending on a phase-matching condition between the pump signal (104) and the frequency-converted optical signal (110). The laser system further comprises a tuning mechanism (108) configured to adjust the phase-match away from an optimal phasematching condition, such that a non-zero phase mismatch is achieved, whereby at least first-order contributions to the RIN of the frequency-converted optical signal (110) are suppressed or reduced.Fig. 2 shows an embodiment of a laser system according to the present disclosure, wherein the laser system comprises a pump laser (202) configured to provide a pump signal (204), a frequency conversion element (206), a tuning mechanism (208), and one or more optical amplifiers (212) configured to amplify the pump signal (204) to generate an amplified pump signal (214). This embodiment is similar to the embodiment described in relation to fig.

[0084] 1, however with the addition of one or more optical amplifiers (212), such as one or more pre-amplifiers and / or one or more high-power optical amplifiers.

[0085] Fig. 3 shows an embodiment of a laser system according to the present disclosure, wherein the laser system comprises a continuous-wave laser (302) configured to provide a continuous-wave pump signal (304), one or more fiber amplifiers (312) configured to amplify the pump signal (304) to generate an amplified pump signal (314), a periodically poled nonlinear waveguide (306), such as a periodically poled lithium niobate (PPLN) waveguide, and one or more temperature controllers (308) configured to adjust a temperature setpoint of the periodically poled nonlinear waveguide (306), whereby a phase mismatch can be achieved, such that the nonlinear waveguide (306) is operated away from an optimal phasematching condition. In other words, the nonlinear waveguide (306) is operated at an operating point of non-maximum conversion efficiency.

[0086] Fig. 4 shows three graphs of the converted power, P2a), versus the nonlinearity, y P^z. The converted power is normalized with respect to the input power such that P2&J / P0is plotted. These graphs are the result of numerical simulations. The situation plotted corresponds to a scenario in which the phase mismatch is zero, i.e., -^= = 0. Accordingly, the pump signal (input signal) and the converted signal are phase matched. The three graphs correspond to three different input powers. In the middle graph, the input power is Po. The input power is then varied a small amount, 8P, to each side of the input power, Po. Thus, the top graph corresponds to an input power of Po+ 8P, and the bottom graph corresponds to an input power of Po- 8P. For simplicity, the horizontal axis may be thought of as the z-position in the frequency conversion element (unitless). It is observed that the converted power increases along the length of the frequency conversion element. The beginning of the three graphs corresponds to a quadratic regime, wherein the converted power can be expressed as P2w= Y2PQ2’ where L is the length of the frequency conversion element. In such cases, there is typically a 6 dB addition of noise to the relative intensity noise of the frequency-converted signal. In case of a phase-matched second-harmonic generation (SHG) conversion process, the converted power in the pump depletion region can be expressed as = Potanh2(yA / PoL). This expression applies to the converted power outside the quadratic regime, and outside the following linear regime; thus, it primarily applies to higher values ofthe nonlinearity, Y P^L, corresponding to z-positions far into the nonlinear crystal, e.g., towards the end of the crystal.

[0087] Fig. 5 shows essentially the same plot as shown in fig. 4, i.e., it shows the converted power versus the nonlinearity, except in this case there is no phase matching of the pump signal and the converted signal. Thus, in this example there is a non-zero phase mismatch, which for illustrative purposes is set such that -^= = 10. Again, in this case the three graphs correspond to three different input powers: Po- 8P, Po, and P0+ 8P. It is observed that without phase matching, the converted power oscillates with the nonlinearity. The converted power can be expressed as P2a)= -^-sin2A / cL). Thus, the period, or conversion length, of the signal is independent of the input power. Thus, the three graphs oscillate with a similar period. The reason for the oscillations is that the direction of energy transfer changes periodically according to the change in the phase relation between the interacting optical signals. In contrast, in a phase matched scenario, the amplitude contributions from different parts of the frequency conversion element adds up along the length of the element, such that the converted power grows with the z-position along the conversion element, cf. Fig. 4.

[0088] Fig. 6 shows essentially the same plot as shown in fig. 4, i.e., it shows the converted power versus the nonlinearity, except in this case there is no phase matching of the pump signal and the converted signal, and furthermore the frequency conversion element is operated in the pump depletion regime. In this example there is a non-zero phase mismatch, which for illustrative purposes is set such that -^= = 1. Again, in this example, the converted power oscillates with the nonlinearity of the conversion element, or correspondingly with the z-position in the element for a given input power and nonlinear coefficient. Similarly, in this figure the three graphs correspond to three different input powers: Po- 8P, Po, and Po+ 8P.

[0089] However, what is noticeably different from the scenarios presented in figures 4 and 5, is that in Fig. 6 the three graphs intersect each other at a single point for a given nonlinearity. In this point, the converted power is independent of the input power, Po. Theoretically, at this point there is an infinite reduction of the relative intensity noise (RIN) of the signal. Thus, at least first-order contributions to the RIN are completely cancelled at this point. In this scenario, the power of the frequency-converted signal can be approximately expressed as

[0090]

[0091] L)- Accordingly, compared to the situation shown in Fig. 5,

[0092]

[0093] there is a contribution to the period of the signal by the term

[0094]

[0095] which is observed to depend on the input power. Therefore, the three curves corresponding to different inputpowers are a bit skewed with respect to each other, since they have different periods. In other words, fluctuations in the power are compensated by a faster oscillation.

[0096] Fig. 7 shows simulated data of the relative power plotted versus time for three different scenarios. The power is shown relative to the average input power, i.e. relative to the average pump power, which is the same in the three scenarios. Starting from the top, the top graph shows the input signal, i.e., the pump signal which is provided as the input to the frequency conversion element. The next graph shows a scenario of phase matching with a relatively high input power. In this case, the contribution or addition to the relative intensity noise (RIN) is about 3.7 dB. The contribution refers to the (intrinsic) contribution from the frequency conversion process. The next graph shows a scenario of phase mismatching, i.e., a nonzero phase mismatch, where there is a large reduction in the RIN of about 17.9 dB. Accordingly, in this case, the RIN of the input signal has been reduced to a large extent. The intensity fluctuations in the signal are seen to be noticeably smaller compared to the other graphs. The bottom graph shows a scenario of phase matching with a relatively low input power. In this case, the contribution or addition to the relative intensity noise (RIN) is about 5.7 dB. The simulated data is provided as an example. Other values of pump power and phase mismatching, and consequently other values of RIN reduction, can be envisaged without departing from the scope of the disclosure.

[0097] Fig. 8 shows the conversion efficiency versus the phase mismatch and nonlinearity. For simplicity, the two independent parameters are provided as unitless numbers. Conversion efficiency may be understood as the ratio of the power of the generated output wave (e.g., the second-harmonic signal) to the power of the input pump signal in a nonlinear optical process. It is typically expressed as a percentage and quantifies how effectively the frequency conversion element converts the input power into the desired output frequency. In this figure, the conversion efficiency takes values from 0 to 1, corresponding to values between 0 % and 100 %. It is observed that the conversion efficiency depends on a phasematching condition between the pump signal and the frequency-converted optical signal, and further that by adjusting the phase mismatch away from an optimal phase-matching condition (corresponding to a phase mismatch of zero), such that a non-zero phase mismatch is achieved, the conversion efficiency is affected. Accordingly, the conversion efficiency can be selected by providing a tuning mechanism configured to adjust the phasematch away from an optimal phase-matching condition. In this context, an optimal phasematching condition refers to a condition in which the conversion efficiency is maximized corresponding to a perfectly phase matched scenario.Fig. 9 shows the relative intensity noise (RIN) contribution in dB (labelled ‘RIN penalty’) versus the phase mismatch and nonlinearity. For simplicity, the two independent parameters are provided as unitless numbers. A RIN contribution may be understood as how much the frequency-conversion process contributes to the RIN of the input optical signal. A positive RIN contribution corresponds to an increase in the RIN. Conversely, a negative RIN contribution corresponds to a reduction of the RIN. It is observed that the RIN contribution oscillates back and forth between negative and positive values for increasing nonlinearity and phase mismatch. Thus, by providing a tuning mechanism for adjusting the phase mismatch away from zero (for a given nonlinearity), the RIN contribution can be affected or controlled. In other words, the laser system may be configured to operate the frequency conversion element in a non-phase matched scenario in order to reduce, or ideally eliminate, the RIN contribution from the frequency conversion process.

[0098] From Fig. 9, it is observed that if the frequency conversion element is operated away from an optimal phase matching of zero phase mismatch, both the conversion efficiency (cf. Fig. 8) and the RIN contribution (Fig. 9) are affected. As a specific example, consider a nonlinearity of Y P^L = 2, corresponding to a vertical line in each plot in figures 8 and 9. If the phase mismatch is increased along this line, e.g., achieved by a temperature detuning of the frequency conversion element, then the conversion efficiency decreases. However, interestingly, the RIN contribution also decreases and at some point crosses zero such that the nonlinear conversion process actually removes RIN from the input signal. Accordingly, the RIN contribution becomes negative. This happens before the conversion efficiency reaches zero. Thus, there exists a compromise where the RIN contribution can be reduced, or entirely removed, at the expense of a lower conversion efficiency. The effect is larger for stronger nonlinearities, e.g., for conversion elements having a high nonlinear coefficient or for long conversion elements.

[0099] Fig. 10 shows an example wherein two graphs are plotted for a given nonlinearity, which in this example is set such that Y P^L = 1.05. The two graphs show the conversion efficiency and the RIN contribution (labelled ‘RIN penalty’) in dB versus the phase mismatch. Theoretically, there exists a phase mismatch wherein the RIN contribution goes to negative infinity, corresponding to a complete cancellation of RIN contributions. The conversion efficiency also decreases for an increasing phase mismatch. In this particular example, the conversion efficiency is quite low at the phase mismatch where the RIN contributions are cancelled (specifically it is about 0.03 in this example). This is because the nonlinearity is selected somewhat low, which implies that the conversion efficiency is relatively low, even for a phase matched condition (here about 0.6 for a phase mismatch of zero). Thus, theexample serves to illustrate that it is preferred to use a relatively high nonlinearity in the conversion process.

[0100] Fig. 11 shows essentially the same graphs as shown in Fig. 10, with the change that the nonlinearity is higher than the foregoing example, specifically it set such that Y P^L = 3.01. The two graphs showing the conversion efficiency and the RIN contribution, respectively, somewhat have similar shapes as in the example of Fig. 10, with the exception that the values are quite different and the compromise in conversion efficiency is much smaller. In other words, since the nonlinearity is higher, the conversion efficiency is higher even for a non-zero phase mismatch, and even in the point wherein the RIN reduction is maximized, corresponding to a negative RIN contribution. The conversion efficiency at this point is observed to have a value of about 0.6, which is significantly larger compared to the previous example of Fig. 10, wherein it was 0.03 at the point of maximum RIN reduction. The example serves to illustrate that it is preferred to use a relatively high nonlinearity in the conversion process.

[0101] Fig. 12 shows experimental data of relative intensity noise (RIN) in units of dBc / Hz. The horizontal axis shows the frequency of the noise in units of Hz. The solid curve shows the RIN of the pump signal, which is the input signal to the frequency conversion element. The two remaining curves, shown as dashed and dash-dotted, show the RIN of the frequency-converted optical signal at phase-matching and non phase-matching, respectively, specifically at optimal phase-matching (dashed), and at a non-zero phase mismatch (dash-dotted). The dashed curved shows the RIN of the frequency-converted signal at a specific operating temperature of the conversion element. In this specific example, the conversion process is a second-harmonic generation (SHG) process, but other nonlinear conversion processes, such as sum-frequency generation (SFG), or difference-frequency generation (DFG), could be considered as well. Generally, the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal.

[0102] In the example of Fig. 12, the optimal conversion efficiency is achieved for a specific temperature, here about 47 °C, which corresponds to a phase matched condition. Thus, the dashed curve corresponds to a phase mismatch of zero. It is observed that, in the phase matched condition, the conversion process adds intensity noise across all noise frequencies, such that the frequency-converted optical signal has a higher RIN level compared to the RIN level of the pump signal, or of the amplified pump signal. In other words, there is a positive RIN contribution from the nonlinear conversion process in the phase matched scenario. The dash-dotted graph shows the RIN of the frequency-converted signal at a non-optimaltemperature, here about 44 °C, wherein a non-zero phase mismatch is achieved by a temperature detuning of the frequency conversion element. It is observed that the temperature detuning causes a significant reduction in the RIN of the frequency-converted optical signal. At least for some frequencies, the RIN reduction is more than 20 dB, such as more than 25 dB. This reduction in the RIN is observed both relative to the RIN of a phase-matched frequency-converted optical signal at the same pump power, and also relative to the RIN of the pump. Furthermore, it is observed that not only is it possible to completely remove the RIN contribution from the frequency-conversion process, but it is also possible to reduce the RIN of the pump signal. The shot noise level is represented as a horizontal line at the bottom of the graph. The specific temperatures are provided only as an example and may be specific to the type of frequency element.

[0103] While the example of Fig. 12 shows the effect using a temperature detuning, more generally this can be achieved by a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved. An optimal phase-matching condition may be understood as a phase-matched process having a phase mismatch of zero. As an example, for a nonlinear crystal, a non-zero phase mismatch could be achieved by rotating the crystal, since the phase-matching condition depends on the incident angle. Furthermore, the effect is even more pronounced when operating the frequency conversion element in a pump depletion regime. In some embodiments, the tuning mechanism is configured to apply an external electric field across the frequency conversion element. The phase mismatch may be adjusted by controlling the amplitude, and / or direction, of the external electric field. Thus, by adjusting the external electric field, a non-zero phase mismatch can be achieved.

[0104] Fig. 13 shows experimental data of the output power (mW) of a frequency-converted optical signal (labelled ‘SHG output power’), for a constant pump power, versus the temperature setpoint (°C) of the frequency conversion element, here embodied as a nonlinear crystal as an example. In this example, the output power increases with increasing temperature of the frequency conversion element until a maximum conversion efficiency is achieved at about 47 °C corresponding to a phase matched condition, wherein there is zero phase mismatch between the pump signal and the frequency-converted signal. Accordingly, both the output power and the conversion efficiency can be adjusted, or selected, by detuning the temperature of the frequency conversion element away from the optimal temperature setpoint (in this example 47 °C). In this particular example, the wavelength of the output optical signal from the conversion is about 785 nm, but other wavelengths can be envisaged without departing from the scope of the disclosure. In other embodiments, the output wavelength from the frequency conversion element is about 780 nm.Fig. 14 shows a graph of the relative intensity noise (RIN) contribution in dB, here labelled as the ‘RIN penalty’, versus the conversion efficiency in percentage, for a fixed pump power. The dots represent experimental data, and the solid curve represents the theoretical RIN contribution. A negative RIN contribution, or a negative RIN penalty, corresponds to a reduction of the RIN. Theoretically, the RIN contribution approaches negative infinity corresponding to a complete cancellation of the RIN. It is observed that the contribution to the RIN decreases in the beginning with increasing conversion efficiency, and then it rises again after a global minimum. Conversely, considering a conversion efficiency at about 55 %, there is a positive contribution to the RIN of about 3 dB, which is reduced as the phase-mismatching is detuned away from optimal phase matching. This detuning causes a reduction of the RIN contribution. As an example, at about 30 % conversion efficiency there is almost no RIN contribution from the frequency conversion process. If a stronger phasemismatch is achieved, the RIN contribution, or ‘RIN penalty’, is even further reduced at the expense of a smaller conversion efficiency. For materials with a larger nonlinearity, such as nonlinear crystals or waveguides with a higher nonlinear coefficient, the conversion efficiency is higher at the maximum RIN reduction. Accordingly, there is a trade-off between the amount of RIN reduction and conversion efficiency; a trade-off which is more favorable for materials exhibiting a high nonlinearity.

[0105] Fig. 15 shows the relative intensity noise (RIN) in dB versus the conversion efficiency in percentage for a range of noise frequencies in Hz. Positive values of the RIN imply that the conversion process adds intensity noise to the input signal, i.e., the pump signal or an amplified pump signal, whereas negative values of the RIN imply that the conversion process removes intensity noise from the input signal. In other words, negative values of the RIN correspond to a reduction in the RIN of the input signal, such that the frequency-converted optical signal has a lower RIN than the input signal. It is observed that for a large range of frequencies, such as frequencies above about 1 kHz, a significant RIN reduction can be achieved, e.g., having values exceeding 24 dB. This is in particular the case for strongly phase-mismatched signals, i.e., wherein the phase mismatch is far removed from zero, which also causes the conversion efficiency to decrease. The example serves to illustrate the trade-off between RIN reduction and conversion efficiency for phase mismatched frequency conversion processes.

[0106] Fig. 16 shows measurements of the contribution from a second-harmonic generation frequency-conversion process to the relative intensity noise (RIN) in dB for a range of noise frequencies in Hz. The experimental data is the same data as shown in Fig. 12; however, visualized differently. In this case, the RIN contribution is shown relative to the RIN of the pump signal (solid graph), said pump signal constituting the input to the SHG frequencyconversion. Thus, the RIN contribution of the pump signal relative to the RIN of the pump signal itself is zero and shown as a horizontal line. The dashed line shows the RIN contribution from the SHG conversion process at a phase-matched condition, i.e., at a temperature setpoint ensuring a zero phase mismatch, or at least a phase mismatch very close to zero. In this particular case, this temperature setpoint corresponds to a temperature of about 47°C. However, the temperature setpoint may generally vary among types and materials of frequency conversion elements. Accordingly, the example should not be construed as limiting the scope of the disclosure. On the contrary, other values can be envisaged without departing from the scope. The dash-dotted curve represents the RIN contribution from an SHG process which is non-phase matched, i.e. wherein there is a nonzero phase mismatch. Accordingly, in this case, the phase-match is tuned away from the optimal phase-matching condition. Specifically, in this example this is achieved by one or more temperature controllers for adjusting the temperature setpoint of the frequency conversion element. In this case, the largest RIN reduction was obtained for a temperature of about 44°C, i.e., a temperature difference of about 3°C from the optimal temperature setpoint corresponding to the optimal phase-matching condition. It is observed that the temperature detuning results in a significant reduction in the RIN of the frequency-converted optical signal, at least for noise frequencies above 1 kHz. In this particular example, a RIN reduction of more than 10 dB is achieved in a range of noise frequencies from about 103Hz to about 2 106Hz. Furthermore, a RIN reduction of more than 20 dB is achieved in a range of noise frequencies from about 105Hz to about 106Hz. While this example is shown for an SHG process, similar results can be obtained from a sum-frequency generation (SFG) process, or a difference-frequency generation (DFG) process.

[0107] Fig. 17 shows the output power (normalized) from a frequency conversion process (labelled ‘SHG output power’) versus the temperature setpoint (in °C) of the frequency conversion element, for a constant pump power. It is observed that there exists a specific temperature setpoint that results in maximum power output, in this case said temperature setpoint is about 47°C. In typical conversion processes, it is desired to set the temperature of the frequency conversion element at a value that maximizes the output power from the conversion element, or at least in the vicinity of this maximum. However, the present inventors have realized that by detuning the temperature away from what is typically considered the optimal temperature setpoint, the relative intensity noise (RIN) of the frequency-converted optical signal can be significantly reduced. In this particular case, it was found that a temperature setpoint of about 44°C resulted in a maximum RIN reduction. Interestingly, this point on the curve does not coincide with a minimum in the output power from the conversion process. Accordingly, there is a trade-off between output power, orcorrespondingly the conversion efficiency, and the amount of RIN reduction. Therefore, the amount of RIN reduction can be selected, within some boundaries such as set by the shot noise limit, by detuning the frequency conversion element away from the temperature setpoint corresponding to the phase-matched condition.

[0108] Fig. 18 shows an embodiment of a quantum computing system according to the present disclosure, said quantum computing system comprising a plurality of qubits (1816), and the laser system disclosed herein. In this embodiment, the laser system comprises a pump laser (1802) configured to provide a pump signal (1804), a frequency conversion element (1806), a tuning mechanism (1808), and optionally one or more optical amplifiers (1812) configured to amplify the pump signal (1804) to generate an amplified pump signal (1814). The plurality of qubits (1816) may comprise trapped atoms or ions. The laser system is configured to deliver the frequency-converted optical signal (1810) to one or more of the qubits (1816). The frequency-converted optical signal may be configured for one or more purposes selected from the group of: cooling of atoms or ions, trapping of atoms or ions, qubit initialization, manipulation of qubit states, and / or combinations thereof. The laser system may be embodied in a variety of ways as described herein.

[0109] Fig. 19 shows an embodiment of a laser system according to the present disclosure. In this embodiment, the laser system comprises a Tm-doped laser (1902) configured to provide a pump signal (1904) having a wavelength in the range from about 1.9 pm to about 2.1 pm, such as at about 2 pm. As an example, the wavelength of the pump signal (1904) could be 2026 nm or 2052 nm. Other wavelengths can be envisioned without departing from the scope of the disclosure. In this embodiment, the laser system further comprises a Tm-doped amplifier (1912) configured to amplify the pump signal (1904) and provide an amplified pump signal (1914). The laser and / or amplifier may be fiber-based components, i.e., a fiber laser and a fiber amplifier, respectively. The laser system further comprises a frequency conversion element (1906), as further described herein, said conversion element (1906) configured to frequency convert the pump signal (1904), or amplified pump signal (1914), by second-harmonic generation (SHG) to provide a frequency-converted optical signal (1910). The second-harmonic generation (SHG) frequency-doubles the input optical signal (1904, or 1914) such that the frequency-converted optical signal (1910) has a wavelength in the range from about 950 nm to about 1050 nm. As an example, the wavelength of the frequency-converted optical signal (1910) may be 1013 nm or 1026 nm. The laser system further comprises a tuning mechanism (1908) configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved. This embodiment may be combined with features from other embodiments, e.g., concerning details of the tuning mechanism (1908) or the frequency-conversion element (1906).An advantage of this embodiment is that it constitutes a low-RIN optical source for providing a low-RIN optical signal at about 1 pm. Thus, this optical source (comprising the Tm-doped laser 1902, Tm-doped amplifier 1912, frequency-conversion element 1906, and tuning mechanism 1908) can be used in other optical systems and laser systems requiring an optical signal having a wavelength of about 1 pm exhibiting low RIN. Any of the advantages described in this disclosure with respect to other embodiments may apply to this embodiment as well. In this embodiment, the laser system may optionally further comprise an Yb-doped amplifier for amplifying the frequency-converted optical signal (1910). Thus, the laser system may be configured to provide an output signal (1918) of about 1 pm, which is amplified, and having a low RIN, compared to the RIN of the pump signal (1904), or the amplified pump signal (1914), and / or compared to a phase-matched condition.

[0110] In summary, the disclosed system and method provide a novel and highly effective approach for generating ultra-low-noise optical signals. By operating a frequency conversion element at a point of non-maximum conversion efficiency, the system achieves the unexpected technical effect of actively reducing the relative intensity noise, generating an output signal that is quieter than the input pump signal. This intrinsic noise-cleaning capability enables the development of laser sources with unparalleled stability, which is particularly advantageous for demanding applications such as quantum computing, precision metrology, and atomic clocks.

[0111] Further details of the disclosure

[0112] 1. A laser system for reducing the relative intensity noise (RIN) of an optical signal, the laser system comprising:

[0113] at least one pump laser configured to provide a pump signal;

[0114] a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal; and

[0115] a tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved.

[0116] 2. The laser system according to item 1 , wherein at least first-order contributions to the RIN of the frequency-converted optical signal are suppressed and / or reduced.3. The laser system according to any of the preceding items, wherein the RIN is reduced at least in a range of frequencies above 1 kHz.

[0117] 4. The laser system according to any of the preceding items, wherein the RIN is reduced at least in a range of frequencies from about 103Hz to about 107Hz, such as from about 104Hz to about 106Hz, such as from about 105Hz to about 106Hz.

[0118] 5. The laser system according to any of the preceding items, wherein a RIN reduction of more than 10 dB is achieved at least in a range of frequencies from about 103Hz to about 106Hz.

[0119] 6. The laser system according to any of the preceding items, wherein a RIN reduction of more than 15 dB is achieved at least in a range of frequencies from about 2 104Hz to about 106Hz.

[0120] 7. The laser system according to any of the preceding items, wherein the RIN reduction is measured relative to the RIN of a phase-matched frequency-converted optical signal at the same pump power.

[0121] 8. The laser system according to any of the preceding items, wherein the RIN reduction is measured relative to the RIN of the pump signal.

[0122] 9. The laser system according to any of the preceding items, wherein the pump laser is a continuous-wave laser or a pulsed laser.

[0123] 10. The laser system according to any of the preceding items, wherein the pump laser is a fiber laser or a solid-state laser.

[0124] 11. The laser system according to any of the preceding items, wherein the power of the pump signal is between 1 mW and 15 W, such as between 1 mW and 10 W, such as between 10 mW and 1 W

[0125] 12. The laser system according to any of the preceding items, wherein the frequency conversion element is configured to frequency convert the pump signal by second- harmonic generation (SHG).13. The laser system according to any of the preceding items, wherein the frequency conversion element is selected among the group of nonlinear crystals or nonlinear optical waveguides.

[0126] 14. The laser system according to any of the preceding items, wherein the frequency conversion element is a periodically poled nonlinear waveguide.

[0127] 15. The laser system according to any of the preceding items, wherein the frequency conversion element is a periodically poled lithium niobate waveguide.

[0128] 16. The laser system according to any of the preceding items, wherein the frequency conversion element is a fiber-coupled device.

[0129] 17. The laser system according to any of the preceding items, wherein the non-zero phase mismatch is achieved by a temperature detuning of the frequency conversion element.

[0130] 18. The laser system according to any of the preceding items, wherein the tuning mechanism comprises one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element.

[0131] 19. The laser system according to item 18, wherein the temperature controller(s) are configured to adjust the temperature setpoint such that a non-zero phase mismatch is achieved.

[0132] 20. The laser system according to any of the items 18-19, wherein the tuning mechanism comprises two or more temperature controllers, wherein the temperature controllers can be controlled individually.

[0133] 21. The laser system according to item 20, wherein the two or more temperature controllers are configured to provide a uniform temperature distribution throughout the frequency conversion element.

[0134] 22. The laser system according to item 20, wherein the two or more temperature controllers are configured to provide one or more temperature gradients, such as at least one temperature gradient, throughout the frequency conversion element.23. The laser system according to any of the items 18-22, wherein the temperature controller(s) comprises a Peltier element to adjust the temperature setpoint of the frequency conversion element.

[0135] 24. The laser system according to any of the preceding items, wherein the laser system further comprises one or more optical amplifiers configured to amplify the pump signal to generate an amplified pump signal.

[0136] 25. The laser system according to item 24, wherein the optical amplifier(s) is arranged downstream of the pump laser and upstream of the frequency conversion element.

[0137] 26. The laser system according to any of the items 24-25, wherein the optical amplifier(s) is a fiber optical amplifier or a semiconductor optical amplifier (SOA).

[0138] 27. The laser system according to any of the items 24-26, wherein the optical amplifier(s) is selected from the group of erbium-doped fiber amplifiers (EDFA), ytterbium-doped fiber amplifiers (YDFA), a thulium-doped fiber amplifiers (TDFA), or erbium-ytterbium co-doped fiber amplifiers.

[0139] 28. The laser system according to any of the items 24-27, wherein the optical amplifier(s) is configured to amplify the pump signal to a power of at least 0.5 W, such as at least 1 W, such as at least 2 W.

[0140] 29. The laser system according to any of the items 24-28, wherein the optical amplifier(s) is configured to amplify the pump signal to a power of at least 5 W, such as at least 10 W, such as at least 15 W.

[0141] 30. The laser system according to any of the items 24-29, wherein the optical amplifier(s) introduces intensity noise in the pump signal, such that the RIN of the amplified pump signal is increased.

[0142] 31. The laser system according to any of the items 24-30, wherein the frequency conversion element is operated in a non-phase-matched regime, such that there is a phase mismatch between the amplified pump signal and the frequency-converted optical signal, whereby the intensity noise introduced by the optical amplifier(s) is reduced or suppressed.32. The laser system according to any of the preceding items, wherein the pump signal and / or the amplified pump signal has a first RIN level depending on the power of the pump signal.

[0143] 33. The laser system according to any of the preceding items, wherein the frequency- converted optical signal has a second RIN level.

[0144] 34. The laser system according to any of the items 32-33, wherein the second RIN level is lower than the first RIN level at least in a range of frequencies above 1 kHz.

[0145] 35. The laser system according to any of the preceding items, wherein the RIN of the frequency-converted optical signal is lower than the RIN of the pump signal at least in a range of frequencies above 1 kHz.

[0146] 36. The laser system according to any of the preceding items, wherein the RIN is measured on a time scale of 10 Hz and above, such as 100 Hz and above.

[0147] 37. The laser system according to any of the preceding items, wherein the RIN of the frequency-converted optical signal is reduced by at least 10 dB, such as at least 20 dB, relative to the RIN of the pump signal, at least in a range of frequencies.

[0148] 38. The laser system according to any of the preceding items, wherein the RIN of the phase-mismatched frequency-converted optical signal is reduced by at least 10 dB, such as at least 20 dB, relative to the RIN of a phase-matched frequency-converted optical signal at the same pump power.

[0149] 39. The laser system according to any of the preceding items, wherein the frequency conversion element is a nonlinear crystal, such as a bulk crystal and / or a birefringent crystal.

[0150] 40. The laser system according to any of the preceding items, wherein the frequency conversion element is periodically poled.

[0151] 41. The laser system according to item 39, wherein the tuning mechanism is configured to adjust the angle of the nonlinear crystal to achieve a non-zero phase mismatch.42. The laser system according to any of the preceding items, wherein the tuning mechanism is configured to apply an external electric field across the frequency conversion element, such as across a nonlinear crystal.

[0152] 43. The laser system according to item 42, wherein the tuning mechanism comprises one or more electrodes for controlling the external electric field, such as for controlling the amplitude of the external electric field.

[0153] 44. The laser system according to any of the preceding items, wherein the laser system comprises two or more pump lasers.

[0154] 45. The laser system according to item 44, wherein the two or more pump lasers comprise a first pump laser for providing a first pump signal at a first pump wavelength and a second pump laser for providing a second pump signal at a second pump wavelength.

[0155] 46. The laser system according to item 45, wherein the frequency conversion element is configured to frequency convert the pump signal by sum-frequency generation (SFG) of the first and second pump signals.

[0156] 47. The laser system according to item 45, wherein the frequency conversion element is configured to frequency convert the pump signal by difference-frequency generation (DFG) of the first and second pump signals, or vice versa.

[0157] 48. The laser system according to any of the preceding items, wherein the power of the frequency-converted optical signal output from the frequency conversion element is at least 100 mW, such as at least 500 mW, such as at least 1 W

[0158] 49. The laser system according to any of the preceding items, wherein the power of the frequency-converted optical signal output from the frequency conversion element is between 1 mW and 10 W, such as between 10 mW and 5 W, such as between 100 mW and 3 W

[0159] 50. The laser system according to any of the preceding items, wherein the frequency conversion element is operated in a pump depletion regime, preferably in a regime of high pump depletion.51. The laser system according to any of the preceding items, wherein the power of the pump signal is sufficiently high that the frequency conversion element is operated in a pump depletion regime.

[0160] 52. The laser system according to any of the preceding items, wherein the nonlinearity of the frequency conversion element is sufficiently high that the frequency conversion element is operated in a pump depletion regime.

[0161] 53. The laser system according to any of the preceding items, wherein the frequency conversion element is sufficiently long such that the frequency conversion element is operated in a pump depletion regime.

[0162] 54. The laser system according to any of the preceding items, wherein the frequency conversion element is operated outside a quadratic regime of converted power versus nonlinearity.

[0163] 55. The laser system according to any of the preceding items, wherein the frequency conversion element is operated in a regime, wherein the power of the frequency- converted optical signal is independent of the power of the pump signal.

[0164] 56. The laser system according to any of the preceding items, wherein, at least to a first order, any RIN contributions to the frequency-converted optical signal are cancelled.

[0165] 57. The laser system according to any of the preceding items, wherein the frequency conversion element is operated in a pump depletion regime and a non-phase- matched regime.

[0166] 58. The laser system according to any of the preceding items, wherein the conversion efficiency of the frequency conversion element is lower compared to a perfectly phase-matched condition.

[0167] 59. The laser system according to any of the preceding items, wherein the laser system is configured to reduce the RIN of the frequency-converted optical signal by detuning the frequency conversion element to achieve a non-zero phase mismatch.

[0168] 60. The laser system according to any of the preceding items, wherein the conversion efficiency changes in response to the adjustment of the phase mismatch.61. The laser system according to any of the preceding items, wherein the laser system comprises an interface for controlling the desired phase mismatch, the amount of RIN reduction, the conversion efficiency, and / or combinations thereof.

[0169] 62. The laser system according to item 61 , wherein an end-user can select the amount of RIN reduction and / or a desired conversion efficiency by interaction with the interface.

[0170] 63. The laser system according to any of the items 61-62, wherein the interface is provided as a graphical user interface.

[0171] 64. The laser system according to any of the items 61-63, wherein the laser system further comprises an electronic display for displaying the interface, such as the graphical user interface.

[0172] 65. The laser system according to any of the preceding items, wherein the laser system is fiber-based, such that at least the pump laser is a fiber laser, and the frequency conversion element is a fiber-coupled device.

[0173] 66. The laser system according to any of the preceding items, wherein the at least one pump laser is a fiber laser for providing a continuous-wave pump signal, and the frequency conversion element is a periodically poled nonlinear waveguide, which is fiber-coupled.

[0174] 67. The laser system according to item 66, wherein the laser system further comprises at least one fiber optical amplifier arranged between the fiber laser and the periodically poled nonlinear waveguide, wherein the fiber optical amplifier is configured to amplify the continuous-wave pump signal.

[0175] 68. The laser system according to any of the preceding items, wherein the laser system further comprises one or more optical fibers for optically connecting at least the pump laser and the frequency conversion element, and optionally further optically connecting at least one optical amplifier arranged between the pump laser and the frequency conversion element.

[0176] 69. The laser system according to any of the preceding items, wherein the frequency conversion element is configured for single-pass frequency conversion processes.70. The laser system according to any of the preceding items, wherein the frequency conversion element is configured for double-pass frequency conversion processes.

[0177] 71. The laser system according to any of the preceding items, wherein the frequency conversion element is not arranged inside an optical cavity.

[0178] 72. The laser system according to any of the preceding items, wherein the laser system does not include a resonant optical cavity arranged to enhance the power of the pump signal within the frequency conversion element.

[0179] 73. The laser system according to any of the preceding items, wherein the frequency conversion element is arranged to receive an input signal, such as a pump signal or an amplified pump signal, and wherein the laser system is configured to transmit the input signal only a single time through the frequency conversion element.

[0180] 74. The laser system according to any of the preceding items, wherein the frequency conversion element is operated in a pump depletion regime, wherein the pump depletion is larger than 75 %, such as larger than 85 %, such as larger than 90 %.

[0181] 75. The laser system according to any of the preceding items, wherein the frequency conversion element has a conversion efficiency that depends on a phase-matching condition.

[0182] 76. The laser system according to any of the preceding items, wherein the tuning mechanism is configured to adjust the phase-match to an operating point of nonmaximum conversion efficiency.

[0183] 77. The laser system according to any of the preceding items, wherein the pump signal has a first RIN level, and the frequency-converted optical signal has a second RIN level, wherein the second RIN level is lower than the first RIN level at least in a range of frequencies.

[0184] 78. The laser system according to item 77, wherein the second RIN level is at least 10 dB lower, preferably at least 20 dB lower, than the first RIN level in a range of frequencies above 1 kHz.79. The laser system according to any of the items 77-78, wherein the second RIN level is lower than the first RIN level at least in a range of frequencies from about 103Hz to about 10sHz.

[0185] 80. The laser system according to any of the preceding items, wherein the tuning mechanism is selected from the group of: temperature controller(s), actuatable stage(s), and / or electrode(s).

[0186] 81. The laser system according to any of the preceding items, wherein the tuning mechanism is selected from the group of: one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element, one or more actuatable stages configured to adjust an angle of the frequency conversion element with respect to a laser beam comprising the pump signal, and / or one or more electrodes configured to apply an external electric field across the frequency conversion element.

[0187] 82. The laser system according to any of the preceding items, wherein the laser system is suitable for quantum applications, such as quantum computing systems, atomic clocks, quantum sensing or metrology, cold-atom interferometry, and / or quantum gravimeters.

[0188] 83. The laser system according to any of the preceding items, wherein the frequency- converted optical signal is suitable for: atom cooling and / or trapping, state preparation of qubits, or precision spectroscopy of atomic / molecular lines.

[0189] 84. A quantum system comprising the laser system according to any of the preceding items, wherein the quantum system is an atomic clock, an atomic interferometer, or a quantum gravimeter.

[0190] 85. A quantum computing system comprising a plurality of qubits and the laser system according to any of the items 1-83.

[0191] 86. The quantum computing system according to item 85, wherein the quantum computing system further comprises a confinement chamber, or vacuum chamber, for holding or storing the plurality of qubits.87. The quantum computing system according to any of the items 85-86, wherein the qubits are neutral atoms or ions.

[0192] 88. The quantum computing system according to any of the items 85-87, wherein the laser system is configured to deliver the frequency-converted optical signal to one or more of the qubits.

[0193] 89. The quantum computing system according to any of the items 85-88, wherein the frequency-converted optical signal is configured for one or more purposes selected from the group of: cooling of atoms or ions, trapping of atoms or ions, qubit initialization, manipulation of qubit states, and / or combinations thereof.

[0194] Although some embodiments have been described and shown in detail, the disclosure is not restricted to such details, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present disclosure. Furthermore, the skilled person would find it apparent that unless an embodiment is specifically presented only as an alternative, different disclosed embodiments may be combined to achieve a specific implementation and such specific implementation is within the scope of the disclosure.

Claims

38Claims1. A laser system for reducing the relative intensity noise (RIN) of an optical signal, the laser system comprising:at least one pump laser configured to provide a pump signal having a given RIN level;a frequency conversion element configured to frequency convert the pump signal to generate a frequency-converted optical signal, wherein the frequency conversion element has a conversion efficiency depending on a phase-matching condition between the pump signal and the frequency-converted optical signal; anda tuning mechanism configured to adjust the phase-match away from an optimal phase-matching condition, such that a non-zero phase mismatch is achieved, wherein a RIN level of the frequency-converted optical signal is lower than the RIN level of the pump signal at least in a range of frequencies.

2. The laser system according to claim 1 , wherein the RIN of the frequency-converted optical signal is lower than the RIN of the pump signal at least in a range of frequencies above 1 kHz.

3. The laser system according to any of the preceding claims, wherein a RIN reduction of more than 10 dB is achieved at least in a range of frequencies from about 103Hz to about 106Hz.

4. The laser system according to any of the preceding claims, wherein a RIN reduction of more than 15 dB is achieved at least in a range of frequencies from about 2 104Hz to about 106Hz.

5. The laser system according to any of the preceding claims, wherein the frequency conversion element is configured to frequency convert the pump signal by second- harmonic generation (SHG).

6. The laser system according to any of the preceding claims, wherein the frequency conversion element is periodically poled, such as a periodically poled nonlinear waveguide.

397. The laser system according to any of the preceding claims, wherein the non-zero phase mismatch is achieved by a temperature detuning of the frequency conversion element.

8. The laser system according to any of the preceding claims, wherein the tuning mechanism comprises one or more temperature controllers configured to adjust a temperature setpoint of the frequency conversion element.

9. The laser system according to claim 8, wherein the temperature controller(s) are configured to adjust the temperature setpoint such that a non-zero phase mismatch is achieved.

10. The laser system according to any of the claims 8-9, wherein the tuning mechanism comprises two or more temperature controllers, wherein the temperature controllers can be controlled individually.

11. The laser system according to any of the preceding claims, wherein the frequency conversion element is a nonlinear crystal, such as a bulk crystal and / or a birefringent crystal, and wherein the tuning mechanism is configured to adjust an angle of the nonlinear crystal to achieve a non-zero phase mismatch.

12. The laser system according to any of the preceding claims, wherein the tuning mechanism is configured to apply an external electric field across the frequency conversion element, such as across the nonlinear crystal.

13. The laser system according to claim 12, wherein the tuning mechanism comprises one or more electrodes configured to control the external electric field, such as configured to control the amplitude of the external electric field.

14. The laser system according to any of the preceding claims, wherein the frequency conversion element is operated in a pump depletion regime, preferably in a regime of high pump depletion.

15. The laser system according to any of the preceding claims, wherein the frequency conversion element is operated in or near an operating point, wherein the power of the frequency-converted optical signal is independent of the power of the pump signal.4016. The laser system according to any of the preceding claims, wherein the frequency conversion element is operated outside a quadratic regime of converted power versus nonlinearity.

17. The laser system according to any of the preceding claims, wherein the conversion efficiency of the frequency conversion element is lower compared to a perfectly phase-matched condition.

18. The laser system according to any of the preceding claims, wherein at least first- order contributions to the RIN of the frequency-converted optical signal are suppressed or reduced.

19. The laser system according to any of the preceding claims, wherein the frequency conversion element is configured for single-pass or double-pass frequency conversion processes.

20. The laser system according to any of the preceding claims, wherein the frequency conversion element is not arranged inside an optical cavity.

21. The laser system according to any of the preceding claims, wherein the frequency conversion element is operated in a pump depletion regime, wherein the pump depletion is larger than 75 %.

22. The laser system according to any of the preceding claims, wherein the RIN of the frequency-converted optical signal is reduced by at least 10 dB relative to the RIN of a phase-matched frequency-converted optical signal at the same pump power.

23. The laser system according to any of the preceding claims, wherein the laser system is suitable for quantum applications, such as quantum computing systems, atomic clocks, quantum sensing or metrology, cold-atom interferometry, and / or quantum gravimeters.

24. The laser system according to any of the preceding claims, wherein the frequency- converted optical signal is suitable for: atom cooling and / or trapping, state preparation of qubits, or precision spectroscopy of atomic / molecular lines.

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