Electro-optical laser-driven light source with electrodeless ignition

Abstract

An electrodeless laser-driven light source includes a laser source that generates CW sustain light and a pump laser that generates pump light. An optical beam coupler combines the CW sustain light and the pump so that the CW sustain light and the pump light propagate collinearly. A Q-switching laser generates pulsed light in response to the pump. A gas-charged bulb is configured to emit high-intensity light from the gas bulb by igniting a pulsed plasma in the breakdown region of the gas bulb with the pulsed light and maintaining a CW plasma in the CW plasma region of the gas bulb with the sustain light, wherein the gas-charged bulb is positioned between the output of the pump laser and the pump input of the Q-switching laser so that the CW plasma turns off the pulsed light generated by the Q-switching laser crystal.

Description

Technology Field

[0001] The section titles used herein are for organization purposes only and should not be interpreted as limiting the subject matter of the claims described in this application in any way. Background Technology

[0002] Numerous commercial and academic applications require high-intensity light across a wide wavelength range. For example, laser-driven light sources are available that provide high intensity across the spectral range from extreme UV through the visible to the infrared region with high reliability and long lifespan. Various examples of these high-intensity light sources are produced by Energetiq, a division of the Hamamatsu Company located in Wilmington, Massachusetts.

[0003] For example, there is an increasing demand for high-intensity light sources in various fields, including biology, chemistry, climate, and physics, for applications such as semiconductor metrology, sensor calibration and testing, shaped light generation, surface metrology, spectroscopy, and other optical measurement applications. As such, there is a need for the development of high-intensity light sources that can improve, for instance, the size, cost, complexity, reliability, stability, and efficiency of these important types of broadband light sources. Brief explanation of the drawing

[0004] The teachings of this invention, according to preferred and exemplary embodiments, are further explained in detail in the following description taken with reference to the accompanying drawings, along with their additional benefits. A person skilled in the art will understand that the drawings described below are for illustrative purposes only. The drawings are not necessarily to scale; instead, the focus is on illustrating the principles of the teachings in general. The drawings are not intended to limit the scope of the applicant's teachings in any way. FIG. 1 illustrates a schematic diagram of an embodiment of a laser-driven light source including electrodeless ignition according to the present teaching. FIG. 2a illustrates an image of a gas-filled bulb of an electrodeless laser-driven light source according to the present teaching, showing emission having only pulsed laser excitation. FIG. 2b illustrates an image of a gas-filled bulb of an electrodeless laser-driven light source according to the present teaching, showing emission having only CW laser excitation. FIG. 3 illustrates the steps of a method for igniting plasma in an electrodeless all-optical laser-driven light source according to the present teaching. FIG. 4a illustrates an example of a Q-switching determination of the present teaching including a gain region and a saturation possible gain / loss region. FIG. 4b illustrates a passive Q-switching laser rod suitable for an electrodeless laser-driven light source according to the present teaching. Figure 5 illustrates a graph of pulse energy and pump current thresholds for a pump laser that generates a laser pulse sufficient to cause gas breakdown as a function of the pulse length of a quasi-CW pump pulse suitable for an electrodeless laser-driven light source according to the present teaching. FIG. 6 illustrates a gas-filled bulb system having a focusing lens assembly suitable for use in an electrodeless laser-driven light source according to the present teaching. FIG. 7 illustrates an electrodeless laser-driven light source according to the present teaching, in which pump light is co-linear with CW laser light, pulsed laser light is projected onto a gas-filled bulb in one plane, and CW laser light is projected onto a gas-filled bulb in a second plane. FIG. 8 illustrates an example of an electrodeless laser-driven light source in which a CW laser light and a pump light are combined in a fiber coupler according to the present teaching, and the pump light, CW laser light, and pulsed light are projected along the same plane into a gas-filled bulb. FIG. 9 illustrates a detailed view of the excitation region in an embodiment of an electrodeless laser-driven light source having collinear laser excitation according to the present teaching. FIG. 10 illustrates an example of a packaged electrodeless laser-driven light source having collinear laser excitation according to the present teaching. Specific details for implementing the invention

[0005] Now, the present teaching will be described in more detail with reference to exemplary embodiments as illustrated in the accompanying drawings. Although the present teaching is described in relation to various embodiments and examples, it is not intended to be limited to these embodiments. On the contrary, the present teaching includes various alternatives, modifications, and equivalents as will be understood by a person skilled in the art. A person skilled in the art approaching the teaching of this application will recognize additional implementations, modifications, and embodiments within the scope of the disclosure described herein, as well as other fields of use.

[0006] In the specification, the designation "one embodiment" or "an embodiment" means that a specific feature, structure, or characteristic described in relation to the embodiment is included in at least one embodiment of this teaching. The appearance of the phrase "in one embodiment" in various places in this specification does not necessarily refer to the same embodiment.

[0007] It should be understood that the individual steps of the method of this teaching may be performed in any order and / or simultaneously as long as the teaching is kept operable. Additionally, it should be understood that the apparatus and method of this teaching may include any number or all of the described embodiments as long as the teaching is kept operable.

[0008] Laser-driven light sources use CW lasers to directly heat gas plasma to the high temperatures required to generate broadband optical light. High-intensity laser-driven light sources offer significant advantages over light sources that use high-voltage electrodes to sustain the plasma. Laser-driven sources rely on photo-discharge plasma, unlike the discharge plasma used, for example, in arc-lamp devices. In discharge lamps, electrode materials can evaporate and alter discharge properties over the lamp's lifespan, which reduces lamp life. Furthermore, electrode-based systems lead to thermal, mechanical, and electrical stress on the light source. While known laser-driven light sources do not rely on electrodes to sustain the plasma, they still use electrodes for plasma ignition.

[0009] Known light sources that rely on electrodes can have significant limitations. For example, electrode-based light sources may have restrictions on lamp head size and constraints on bulb mounting methods. Electrode-based light sources must be designed to avoid parasitic arcs, and the lamp head needs to be constructed with sufficient volume for the electrodes and ignition circuit. Electrode-based light sources have limitations on the cold charging pressure of the bulb, for example, because the glass-to-metal sealing of the electrodes can limit the maximum charging pressure. Additionally, the size of the bulb in electrode-based light sources may increase, which can affect the bulb charging pressure. Electrode-based light sources also limit the bulb shapes that can be accommodated. This is because, for example, electrode-based light sources require positioning, fixing, and connecting the electrodes. These design constraints can lead to noise generation in the light source.

[0010] Therefore, providing a laser-driven light source with electrodeless ignition can lead to improved reliability, performance, reduced cost, and complexity in addition to other advantages. Igniting plasma with optical illumination requires careful design and control of the light source and associated light delivery mechanisms used for plasma ignition. One feature of the present teaching is to provide a laser-driven light source with electrodeless ignition. In these sources, the plasma is ignited by optical illumination rather than by electrical energy provided by electrodes, as in known laser-driven high-intensity light sources.

[0011] There are numerous features and advantages to electrode-free laser-driven light sources. Electrode-free light sources can be implemented using smaller bulbs with higher maximum charge pressures than conventional light sources. Higher charge pressures can result in higher brightness, particularly in certain laser power schemes. Electrode-free light sources are free from contamination from electrode materials. Additionally, there are fewer geometric restrictions on lamp shapes. Generally, smaller lamp heads can be used for the same characteristics. Furthermore, there are no high-voltage active electrical components to supply power, which reduces the need for associated power supplies, control electronics, and electrical connections, thereby significantly reducing the number of required components. However, some embodiments of electrode-free laser-driven light sources can be implemented in existing lamp packages of laser-driven light sources with electrode ignition. This is at least in part because electrode-free devices are generally less complex and smaller than electrode-based laser-driven light sources. For example, electrodeless laser-driven light sources are described in U.S. Patent Application No. 17 / 328,433, titled “Laser-Driven Light Source with Electrodeless Ignition,” which is assigned to the assignee and incorporated herein by reference.

[0012] One feature of this teaching is that it can provide an electrodeless laser-driven light source without the need to electronically control separate ignition optical pulses. Rather, the optical ignition pulses are generated all-optically within the source components. This approach offers several advantages over electrodeless laser-driven light sources where the pulse source is electrically controlled. For example, much smaller bulbs can be used. Additionally, the number of electronic connections to the source package can be reduced. Furthermore, specialized power supplies or laser-driving electronics are not required to form an electronically driven and controlled pulse source.

[0013] FIG. 1 illustrates a schematic diagram of an embodiment of a laser-driven light source (100) including electrodeless ignition according to the present teaching. A pump laser (102) supplies optical pump light (104) to a gas cell (106). The pump light appears as pass-through pump light (108) that passes through the cell and is supplied to a Q-switch crystal (110). The pass-through pump light (108) coming from the gas cell (106) has appropriate energy that causes the Q-switch crystal (110) to generate optical pulse light (112) that is supplied back to the gas cell (106). The energy from the optical pulse light (112) is supplied to the plasma breakdown region of the gas cell (106) to ignite the plasma. In some embodiments, the gas cell (106) is a gas-filled bulb. In some embodiments, the gas cell (106) is a gas chamber comprising at least one window that passes wavelengths of pump light (104), optical pulse light (112), CW light (114) and high-intensity CW plasma light (118).

[0014] The CW laser (114) generates CW laser light (116). The CW laser light (116) is provided to the gas cell (108) as sustain light for the plasma breakdown region of the gas cell (108). The region of the gas cell (106) illuminated by the pulsed laser light (112) may be referred to as the pulsed illumination region and has a well-defined position and shape based on optical projection elements (not shown) used to direct the pulsed light (112) from the Q-switch crystal (110) and the CW laser light (116) toward the cell (106). These projection elements include various focusing and / or directing elements in various different embodiments. The energy from the optical pulsed light (112) provided to the plasma breakdown region of the cell (106) ignites the plasma. The area illuminated by the CW sustain light may be referred to as the CW sustain light area and has a well-defined position and shape based on projection elements not shown used to direct the optical light from the CW laser (114). These projection elements include various focusing and / or directing elements in various different embodiments. Energy from the CW laser light (114) provided to the plasma breakdown area sustains the plasma. The plasma breakdown area generates high-intensity CW plasma light (118).

[0015] The optical pulse light (112) has the high-peak-power required to ignite the plasma in the gas cell (106). However, to generate the sustained CW plasma light (118), the optical pulse light (112) must not exceed a specific energy / power threshold, which may occur when the optical pulse is present in the plasma after a predetermined delay following the plasma light (118) reaching a predetermined threshold. Too much energy from the optical pulse light (112) delivered to the gas cell (106) can extinguish the plasma. To prevent the optical pulse light (112) from extinguishing the plasma, a mechanism is required to extinguish the pulse light (112) after the generation of the sustained CW plasma light. This can be implemented in an all-optical laser-driven light source (100) by utilizing the pump laser light reduction of the gas cell (106) that occurs when the plasma is ignited. When the plasma is ignited, the pump light (108) passing through the cell and provided to the Q-switch crystal (110) has significantly reduced power compared to the pump light (104) coming from the pump laser (102). Additionally, when the plasma is ignited, the pump light passing through the cell (106) and coming out through the pump light (108) has significantly reduced power compared to the pump light passing through the cell (106) and coming out through the pump light (108) when the plasma is not ignited. This extinguishing through the pump light (108) eventually extinguishes the pulsed laser light (112). By extinguishing the pulsed light (112) before the subsequent pulse extinguishes the CW plasma, the plasma light can be maintained by applying only the CW sustaining light (116) from the CW laser (114).

[0016] Since the pulsed light (112) is stopped when the CW plasma is ignited, there is no need for a sensor and / or an electronic shutter and / or other electronic blocker to actively extinguish the pulsed light (112) after breakdown. If the plasma is extinguished for some unexpected reason, it will automatically start attempting pulse ignition again because the plasma is no longer present to absorb the pump light. That is, the plasma will re-ignite itself. This results in a simple, compact, and highly reliable electrodeless source. Active monitoring is not required to control the pulsed light.

[0017] One feature of the embodiments of the all-optical electrodeless source of this teaching is that the positions of the pulsed illumination region and the CW sustained illumination region can be adjusted to provide the desired performance of the plasma light. The relative positions of the pulsed illumination region and the CW sustained illumination region affect the threshold of energy / power over which the CW sustained plasma is exceeded. In some embodiments, the two regions are distinct and do not overlap. In some embodiments, the two regions overlap to a small extent.

[0018] FIG. 2a illustrates images (200) of a gas-filled bulb of an embodiment of an electrodeless laser-driven light source according to the present teaching, illustrating emission having only pulse laser excitation. Both images (202, 204) are shown from a side view, but are angle-off to illustrate the three-dimensional positioning of the focus of the CW laser and the locations of the pulsed illumination region. The range and location of the pulsed illumination region can be seen in these images (202, 204). Note that the location of the pulsed breakdown depends on the pulse energy. As the energy increases, the breakdown location moves toward the pulsed laser. The three-dimensional alignment of the pulsed laser breakdown plasma with respect to the pulsed light and the focus of the CW laser enables the method to work with lower CW laser power and / or lower pulse energy.

[0019] FIG. 2b illustrates images (250) of the gas-filled bulb shown in FIG. 2a, illustrating emission having only CW laser excitation. Two images (252, 254) are shown from a side view, but are angle-off to illustrate the three-dimensional positioning of the focus of the CW laser and the location of the pulsed illumination area. The range and location of the pulsed illumination area can be seen in these images (252, 254). Outlines (256, 258) of the pulsed illumination areas from the images (202, 204) of FIG. 2a are also illustrated. In this embodiment, the relative positions and shapes of the pulsed illumination area and the CW sustained illumination area are such that the two areas are distinct and do not overlap. The pulsed illumination area becomes smaller as the light is more densely focused, allowing high-density pulsed energy to be delivered to the plasma by the illumination.

[0020] The images in Figs. 2a and 2b were collected during experiments to determine the operating parameters for the pulses providing plasma ignition. Some details of the experimental conditions are described below. For example, stable plasma ignition in a xenon gas-filled bulb can be realized with a pulse rate of 2 kHz and a pulse duration of 1 ns, across an energy range of 135 to 225 microjoules. The pulsed light had a wavelength of 1064 nm. In this specific example of the experimental setup, the critical energy for breaking down the light to achieve a stable CW plasma was 135 microjoules. Additionally, at 210 microjoules, the plasma can ignite and eventually stabilize, although there may be ignition and extinction of the CW plasma prior to stable operation. It is also possible to realize ignition above 225 microjoules. The relative position of the pulsed light illumination and the CW illumination is important. Adjusting the alignment of the bulb and the CW laser along the optical axis of the pulsed laser improves or 'turns off' the ignition. In these experiments, after the CW plasma was ignited, the CW laser power could be reduced to 8 to 10 watts while still maintaining the CW laser power. At any value of CW laser power exceeding 15.5 watts, a reliable transition from pulsed to CW was achieved. There was no upper limit to the experienced CW laser power. As understood by a person skilled in the art, beam quality affects the energy delivered to the gas for a given laser power.

[0021] One feature of the electro-optical electrodeless laser-driven light source of the present teaching is that the pulsed laser light can be extinguished by the absorption process of the gas cell rather than relying on external electronic feedback. FIG. 3 illustrates the steps of a method for igniting plasma in an electrodeless electro-optical laser-driven light source according to the present teaching. In the first step (302), electromagnetic energy is provided to the gas in the bulb using continuous wave CW laser light (e.g., light (116) from the laser (104) of FIG. 1 to the gas cell (106)). In the second step (304), laser pump radiation is provided to a Q-switched crystal (e.g., light (108) from the laser (112) of FIG. 1 to the crystal (116)). The laser pump radiation causes the Q-switched crystal to generate pulsed light having a specific pulse duration and pulse repetition rate that depends on the power of the laser pump radiation input to the crystal. In the third step (306), the generated pulsed light is provided to the gas (e.g., light (112) provided to the cell (106) of FIG. 1). In some embodiments, the gas is contained in a pressurized bulb. In some embodiments, the gas is xenon gas. The provided pulsed laser light generates breakdown in a specific region of the gas, a breakdown region. In the fourth step (308), ions and electrons in the breakdown region are generated at a density sufficient to seed the CW plasma by absorbing the provided CW laser light. In the fifth step (310), the electromagnetic energy provided by the CW laser light generates a CW plasma that emits high-intensity light in the plasma region. In the sixth step (312), the generation of the CW plasma extinguishes the pulsed light generated by the Q-switching crystal. In some embodiments, the extinguishing is the result of the absorption of pump light passing through the gas having the ignited plasma before colliding with the Q-switching crystal.In step 7, high-intensity light is maintained by using CW laser light to maintain the plasma in a bulb or other container after the pulsed laser light has been extinguished. Note that in some embodiments, the CW continuous light is a continuous light source generated by pulsed laser operation at a nominal high pulse rate. In some configurations, the laser maintaining the CW plasma may also be the same laser pumping the nonlinear crystal.

[0022] Various embodiments of laser-driven high-intensity sources having electrodeless ignition according to the present teaching use different parameters of light supplied to a gas. For example, the repetition rate of Q-switching laser pulses can be controlled. The pulse energy of the pulsed light supplied to the gas can be controlled. The duration of the Q-switching laser pulses can also be controlled. Additionally, the power of the CW laser light is also controlled. In some embodiments, the pulse repetition rate of the pulsed laser light is in the range of 1 kHz to 20 kHz.

[0023] Experimental and / or theoretical evaluations have determined, for example, that high-quality CW plasma can be provided when the Q-switched laser crystal is configured such that the pulse repetition rate of the pulsed laser light is 1 kHz or less. Continuous wave plasma can be generated when the Q-switched laser crystal is configured such that the pulse energy of the pulsed laser light is in the range of 50 microjoules to 500 microjoules.

[0024] Continuous wave plasma is generated under various pulse energy, pulse duration, and CW power conditions depending on the specific configuration. For example, continuous wave plasma is generated when a Q-switched laser crystal is configured such that the pulse energy of the pulsed laser light is in the range of 500 microjoules to 5 millijoules. Additionally, continuous wave plasma can be generated when a Q-switched laser crystal is configured such that the pulse duration of the pulsed laser light is in the range of 0.1 ns to 10 ns. Continuous wave plasma can also be generated when a CW laser source is configured such that the power of the CW sustained light is in the range of 5 W to 50 W. Additionally, continuous wave plasma can be generated when a CW laser source is configured such that the power of the CW sustained light is in the range of 5 W to 1500 W. The aforementioned ranges are merely examples of operating ranges and are not intended to limit the teaching in any way.

[0025] One feature of the present teaching is that different known Q-switching crystals may be used. The wavelength of the pulsed light must be suitable for causing breakdown of the gas species(s) of the bulb. FIG. 4a illustrates an example of a Q-switching crystal (400) of the present teaching comprising a gain region (402) and a saturable gain / loss region (404). As understood by a person skilled in the art, different host materials and dopants may be used to provide a suitable gain region (402) and a saturable absorber region (404). For example, the crystal (400) may have a host material that may be a glass host, a yttrium aluminum garnet host, or a spinel host. For example, the crystal (400) may have a dopant that may be an ytterbium dopant, a chromium dopant, a cobalt dopant, or a vanadium dopant in one or both of the gain region (402) and the saturable absorber region (404). The Q-switch crystal may also include a narrowband filter that can be used, for example, to reflect at least a portion of the plasma light or to block wavelengths of the xenon spectrum. For example, such a coating may be a protective coating, a reflective coating, and / or an anti-reflective coating.

[0026] FIG. 4b illustrates a passive Q-switching laser rod (450) suitable for an electrodeless laser-driven light source according to the present teaching. The yttrium aluminum garnet-based (YAG-based) passive Q-switching laser rod (450) has a curved surface (452). The saturable absorber region (454) is a chromium dopant of the yttrium aluminum garnet host. The gain region (456) is a ytterbium dopant of the yttrium aluminum garnet host. The dopant and host contribute to setting the wavelength of the pulsed light as well as the rise and fall times of the pulses. The saturable absorber region length (458), L2, the gain region length (460), L1, and the crystal width (462), W, are selected to provide desired output pulse parameters, such as pulse repetition rate, pulse duration, and pulse energy.

[0027] Q-switched crystals are a proven technology. For example, Q-switched crystals are used in known passive Q-switching microchip lasers. As one specific example, a microchip laser using a crystal with a saturable absorber region length (458), L2 = 1.36 mm, a gain region length (460), L1 = 3 mm, and a crystal width (462), W = 3 mm provided a 1.6 ns pulse with an energy of 74 microjoules at a repetition rate of 14 kH, which was realized from a 970 nm wavelength pump laser with a pump power of 10 W. As the pump power increases, the average output power and the generated pulse repetition rate can increase to 1 W and 13.6 kHz, respectively, for a pumping power of 9.3 W. Maximum output power can be reached without observable thermal roll-over. An average pulse width of 1.58 ± 0.04 ns can also be realized. In fact, pulse energy and peak power values ​​of 73.8 ± 0.7 μJ and 46.0 ± 0.8 kW, respectively, were realized. One feature of the present teaching is that electrodeless ignition can be realized with pulse light parameters that can be realized by sources of such high availability, compact, and reliable optical pulses provided by Q-switch crystals (400, 450) pumped by optical pump light.

[0028] Pumping efficiency and pulse output depend on various properties of the crystal (400, 450), including gain crystal (402, 456), doping element (e.g., YB or Nd), doping percentage, diameter, and length. For saturable absorber crystals (404, 452), there is a doping element (e.g., Cr or V), doping percentage, initial absorption percentage, diameter, and / or length. In some embodiments, reflective and / or transmissive coatings for pump wavelength and pulsed light wavelength are provided on one or more ends of the crystal (400, 450). For example, a Yb:YAG-Cr:YAG coupled crystal may include a coating on the Yb:YAG end that is highly transmissive at 940 nm and highly reflective at 1030 nm. And, on the Cr:YAG end, the crystal may have a coating that is partially reflective at 1030 nm (i.e., output coupler). While many Q-switching lasers have pump configurations with a saturable absorber and an output coupler on the opposite end of the incoming pump laser, pulsed Q-switching crystals for electrodeless ignition may have an output coupler on the pump input end rather than on the saturable absorber end.

[0029] Some embodiments of the crystals (400, 450) may have undoped end sections around Yb:YAG or Cr:YAG that can be referred to as non-absorbing mirrors. This configuration avoids thermal overload and facet errors. In the gain region (402, 456), Nd:YAG gain media are common and relatively low cost. The Nd:YAG gain region (402, 456) is pumped at 808 nm and emits light at 1064 nm. The Yb:YAG gain region (402, 456) is less common and more expensive. These materials are pumped at a wavelength of 940 or 970 nm and emit light at 1030 nm. These Yb:YAG crystals are most commonly coated to accommodate 940 nm pumping. Crystals coated for 940 nm may not function well at 970 nm (for example, a coating at 940 nm transmits only 60% at 970 nm). Additionally, light at a wavelength of 940 nm is generally easier to separate from 1030 nm than light at 970 nm. It is also possible to pump Yb-doped glass at 975 nm. This pump wavelength is the same as that used for known laser driving light source wavelengths.

[0030] Some important features of the design of a Q-switched crystal for generating pulsed light for electrodeless ignition according to the present teaching include, for example, the selection of the laser wavelength, the coating array sequence, the gain section, the saturable absorber section, and the direction of the pump pulse input and output. Other important features include the coupling / separation of the pump and plasma beams and the accommodation of the need to protect the CW laser from the pulses generated by the Q-switched crystal. Referring again to FIG. 1, different embodiments of the light source (100) may have different configurations for the positions of the pump laser (102), Q-switched crystal (116), and CW laser (114) that affect these design choices. Additionally, due to the high pulse energies, the mounting of the crystal (116) and associated thermal management are important considerations.

[0031] FIG. 5 illustrates a graph (500) of pulse energy and pump current thresholds for a pump laser that generates a laser pulse sufficient to cause gas breakdown as a function of the pulse length of a quasi-CW (QCW) pump pulse used in an embodiment of an electrodeless laser-driven light source according to the present teaching. That is, the pulse length is the width of the repetitive pulse used to generate the quasi-CW pump light signal (e.g., the pulse width of a square wave signal). Graph (500) shows measurements for a bulb containing xenon gas. Graph (500) illustrates an exemplary operating point and shows that operation may occur over various pulse durations. The threshold level out when the pulse length is longer than ~500 microseconds. It should be noted that various embodiments of the light source of the present teaching may operate with different parameters than those illustrated in this exemplary data. Some examples of operating parameters for pulse ignition and transition handoff for xenon gas 22 atm low-temperature bulb charging are as follows: (1) CW transition handoff can be realized at a CW laser power as low as 14 W at a wavelength of 980 nm; (2) CW transition handoff can be realized at a CW laser light center wavelength as low as 972 nm; (3) Nearly instantaneous CW transition handoff can be realized at a CW laser light center wavelength as low as 975 nm; and (4) CW transition handoff can be realized at a CW laser power as high as 50 watts. When the laser spectral content at 980 nm goes to zero, the transition handoff may take several seconds to 1 or 2 minutes. When the CW laser power is 20 watts, for a successful transition handoff, the variable center wavelength is 1 to 2 nm away from the 980 nm center wavelength. With a 30 atm low-pressure cryo-charge bulb, a CW transition handoff from a CW laser at a center wavelength of 30 W and 976 nm can be achieved. Generally, ignition is more robust in a high-pressure bulb than in a low-pressure bulb.For example, a light bulb with a pressure of more than 30 atm will generally have a more solid ignition than a light bulb with a pressure of about 22 atm.

[0032] FIG. 6 illustrates a gas-filled bulb system (600) having focusing lens assemblies (604, 606) suitable for use in an electrodeless laser-driven light source according to the present teaching. A plasma region (608) is illustrated. The focusing lens assemblies (604, 606) are configured in planes oriented 90 degrees relative to each other. One assembly (604) directs pulsed light into the plasma region (608) of the bulb (602), and the other assembly (606) directs CW sustaining light into the plasma region (608) of the bulb (602). As described herein, the shapes of the pulsed light and the CW sustaining light in the plasma region (608) may be the same or different. The positions of the pulsed light and the CW sustaining light in the plasma region (608) may overlap or be separate. In some embodiments, the bulb (602) is filled with xenon gas. In some embodiments, the bulb (602) is formed in a spherical shape. Also, in some embodiments, the pressure of the gas-filled bulb (602) may be a pressure in the range of 20 atm to 50 atm. Referring again to FIG. 5, a graph of pulse energy and pump current thresholds for a pump laser that generates a laser pulse sufficient to cause gas breakdown as a function of pulse length for a passive Q-switching laser crystal suitable for an electrodeless laser-driven light source according to the present teaching is illustrated.

[0033] FIG. 7 illustrates an electrodeless laser-driven light source (700) in accordance with the present teaching, wherein pump light (702) is collinear with CW laser light (704), pulsed laser light (706) is projected onto a gas-filled bulb (not shown) in one plane, and CW laser light (704) is projected onto a gas-filled bulb in a second plane. A pump laser (708) generates pump light (702). In some embodiments, pump light (702) is continuous light having a specific average power. A CW laser (710) generates CW light (704). The CW laser (710) may be a fiber laser, and the light (704) is transmitted into free space through an optical fiber (712) using a collimation package (714). The pump light (702) has a wavelength λp, the CW light (704) has a wavelength λCW, and the pulsed laser light (706) has a wavelength λpulse. A dichroic mirror (716) is used to combine the output light from the CW laser (710) and the pump laser (708) into a collinear propagation. An optional directional element (718) may be used to direct light from the collimation package (714) to the dichroic mirror (716). In some embodiments, the mirror (716) is a long-pass transmission component with high transmittance at about 900 nm and high reflectance at about 900 nm. An optional directional mirror (720) may be used to project the collinear propagation pump light (702) and the CW laser light (704) onto a focusing element (724) and a plasma holding area (722). The second focusing element (726) focuses the pulsed light (706) from the Q-switching crystal (728) to the breakdown region (730). The focusing element (726) also collimates and / or refocuses the pump light passing through the gas (732) onto the Q-switching crystal (728). The drawing illustrates in this perspective view that the plasma holding region (722) and the plasma breakdown region (730) overlap.However, the angles and / or planes and beam shapes of incidence of the CW light (704) and pulsed light (706) can be adjusted so that the regions (730, 722) do not overlap or only partially overlap in the three-dimensional region of the gas. High-intensity light (734) comes from the CW plasma maintained by the CW sustaining light (704).

[0034] FIG. 8 illustrates an embodiment of an electrodeless laser-driven light source (800) in which a CW laser light (802) and a pump light (804) are combined in a fiber coupler (820) according to the present teaching, and the pump light (804), CW laser light (802), and pulsed light (806) are projected along the same plane into a gas-filled bulb (810). The pump laser (812) has a fiber output (814). In some embodiments, the pump laser (812) is a fiber laser. In some embodiments, the pump laser (812) is a fiber-coupled diode or other solid-state laser. The CW laser (816) has a fiber output (818). In some embodiments, the CW laser (816) is a fiber laser. In some embodiments, the CW laser (816) is a fiber-coupled diode or other solid-state laser. In some embodiments, the CW laser (816) is a pulsed laser having a sufficiently high pulse rate to have a semi-continuous output light.

[0035] Light from fibers (814, 818) at the output of the pump laser (812) and the CW laser (816) is combined in a fiber beam combiner (820). In some embodiments, the fiber beam combiner (822) is a fusion fiber combiner. The fiber output (822) of the fiber beam combiner (820) provides combined CW laser light (802) and pump light (804), and the combined light is collimated in a collimator (824). In some embodiments, the collimator (824) collimates light of a nominal wavelength of 980 nm from a ~100 micron fiber core to form a ~8 to 10 mm collimated beam. In some embodiments, the collimator (824) collimates light of a nominal wavelength of 808 nm from a ~100 micron fiber core to form a ~2 to 3 mm collimated beam. In some embodiments, the collimator (824) is a gradient index (GRIN) lens. An optional filter (826) may be used to prevent pulsed light (806) from entering either or both of the pump laser (812) or the CW laser (816).

[0036] Optional directional elements (828) and focusing elements (830), such as mirrors, are used to directional and / or focus CW light (802) and pump light onto a gas-filled bulb (810). The focusing element (832) is used to directional and / or collimate the pass-through pump light (834), which is the pump light (804) passing through the gas-filled bulb (810), onto a Q-switching crystal (836). The pass-through pump light (806) causes the Q-switching crystal (836) to generate pulsed light (806). The collimating element (832) projects the pulsed light (806) onto the gas-filled bulb (810).

[0037] A breakdown region is formed in the gas-filled bulb (810) where pulsed light (806) energy generates a breakdown of the gas. From the breakdown region of the bulb (810), a sufficient density of ions and electrons absorb the CW light (802) of the bulb and seed the CW plasma. The CW plasma emits high-intensity light (838) maintained by the CW light (802). The generation of plasma in the bulb causes a reduction in the power of the pass-pump light (806). This extinguishes the generation of pulsed light (806) from the Q-switch crystal (836).

[0038] One feature of the present teaching is that it is possible to support collinear propagation of all pump, CW, and pulsed light beams and to have focal positions of multiple wavelengths of these beams arranged to overlap in a plane. FIG. 9 illustrates a detailed view of the excitation region in an embodiment of an electrodeless laser-driven light source (900) having collinear laser excitation according to the present teaching. A pump beam (902) and a CW light beam (904) are incident on a lens (906). In some embodiments, the wavelength of the pump beam (902) is 808 nm. In some embodiments, the wavelength of the CW laser light beam (904) is 980 nm. In some embodiments, the lens (906) is an aspherical lens. In some embodiments, the lens (906) is a lens with a nominal aperture of 0.5 inches, f = 10 mm (wavelength dependent), and NA = 0.55. The lens (904) focuses the incoming optical beam of the CW light wavelength at the focal plane (908, B). The plasma holding region (910) is also located on this plane (908). The second lens (912) is located at a certain distance from the plane (908, B) at the same focal length as the lens (912) at the wavelength of the pulsed light (914). In some embodiments, the wavelength of the pulsed light (914) is 1064 nm. Therefore, even if the same lens element model is used for the lenses (906, 912), the position of the first lens (906) from the plane (908, B) and the position of the second lens (912) from the plane (908, B) are different because the focal lengths of the lenses (906, 912) are a function of wavelength. In some embodiments, the width of the CW light beam (904) is greater than the width of the pump beam (902), which is greater than the width of the pulsed light beam (914). Accordingly, the focused spot size of the CW light beam (904) is smaller than the spot size of the pump beam (902), which is smaller than the spot size of the pulsed light beam (914) in the focal plane (908, B). In some embodiments, the width of the CW light beam (904) is nominally 9 mm, the width of the pump beam (902) is nominally 3 mm, and the width of the pulsed light beam (914) is nominally 1 mm.In various embodiments, the spot size and / or beam width of the pulsed light beam (914) is adjusted based on the desired energy density required in the plasma breakdown region.

[0039] One feature of the all-optical electrodeless laser-driven light source of the present teaching is that it can be configured to fit into a single optical package. FIG. 10 illustrates an embodiment of a packaged electrodeless laser-driven light source (1000) having collinear laser excitation according to the present teaching. A high-pressure electrodeless xenon gas-filled bulb (1002) is positioned in the package (1004). A focusing lens (1006) is used to focus the pump light generated by the pump laser diode (1008) and the CW light generated by the laser diodes (1010, 1012). The light from the laser diodes (1008, 1010, 1012) is collimated by a collimation optical system (1014, 1016, 1018). The pump light is directed by a mirror (1020), and the CW light is combined and collinearized with the pump beam path by dichroic elements (1022, 1024). A filter (1026) allows the pump light and CW light to pass through but blocks the pulsed light. A focusing lens (1028) focuses the pulses generated in the Q-switching crystal (1030) into the breakdown area of ​​the xenon gas-filled bulb (1002). The Q-switching crystal includes a Yb:YAG rod section and a Cr:YAG or V:YAG saturable absorber section (1034). A high-reflection coating (1036) is applied to the ends of the crystal (1030).

[0040] Equivalents

[0041] Although the applicant's teaching has been described in relation to various embodiments, the applicant's teaching is not intended to be limited to these embodiments. On the contrary, the applicant's teaching includes various alternatives, modifications, and equivalents, which may be made herein without departing from the spirit and scope of the teaching as understood by a person skilled in the art.

Claims

Claim 1 As an electrodeless laser-driven light source, a) a sustaining laser source that generates continuous wave (CW) sustaining light at the output; b) a pump laser that generates pump light at the output; c) an optical beam combiner having a first input optically coupled to the output of the sustaining laser source and a second input optically coupled to the output of the pump laser, and combining the CW sustaining light and the pump light at the output so that the CW sustaining light and the pump light propagate collinearly; d) a Q-switching laser crystal having a pump input optically coupled to the output of the pump laser, and configured such that the Q-switching laser crystal generates pulsed light at the output in response to the pump light; and e) a gas-charged bulb optically coupled to the output of the sustaining laser and optically coupled to the output of the Q-switching laser crystal so as to emit high-intensity light from the gas bulb by the pulsed light igniting a pulsed plasma in the breakdown region of the gas bulb and the sustaining light sustaining a CW plasma in the CW plasma region of the gas bulb, wherein the gas-charged bulb is positioned between the output of the pump laser and the pump input of the Q-switching laser crystal so as to turn off the pulsed light generated by the Q-switching laser crystal by the CW plasma absorbing the pump light, an electrodeless laser-driven light source. Claim 2 An electrodeless laser-driven light source according to claim 1, wherein the holding laser source and the pump laser comprise the same laser. Claim 3 An electrodeless laser-driven light source according to claim 1, wherein the holding laser source and the pump laser are configured in a single laser housing. Claim 4 An electrodeless laser-driven light source according to claim 1, wherein the optical beam coupler comprises a dichroic beam splitter. Claim 5 An electrodeless laser-driven light source according to claim 1, wherein the optical beam coupler comprises a fiber coupler. Claim 6 The electrodeless laser-driven light source of claim 1, wherein the Q-switching laser crystal is configured such that the pulse repetition rate of the pulsed light is in the range of 1 kHz to 20 kHz. Claim 7 The electrodeless laser-driven light source of claim 1, wherein the Q-switching laser crystal is configured such that the pulse repetition rate of the pulsed light is in the range of 1 kHz or less. Claim 8 The electrodeless laser-driven light source of claim 1, wherein the Q-switching laser crystal is configured such that the pulse energy of the pulsed light is in the range of 350 μJ (Joule) to 50 mJ. Claim 9 An electrodeless laser-driven light source according to claim 1, wherein the Q-switching laser crystal is configured such that the pulse duration of the pulsed light is in the range of 0.1 ns to 10 ns. Claim 10 The electrodeless laser-driven light source of claim 1, wherein the sustaining laser source is configured such that the power of the CW sustaining light is in the range of 5 W to 50 W. Claim 11 The electrodeless laser-driven light source of claim 1, wherein the sustaining laser source is configured such that the power of the CW sustaining light is in the range of 5 W to 1500 W. Claim 12 The electrodeless laser-driven light source according to claim 1, wherein the Q-switching laser crystal comprises a gain section and a saturable absorber section. Claim 13 The electrodeless laser-driven light source according to claim 1, wherein the Q-switching laser crystal comprises at least one of a glass host, a yttrium aluminum garnet host, and a spinel host. Claim 14 The electrodeless laser-driven light source according to claim 1, wherein the Q-switching laser crystal comprises at least one of a chromium dopant, a cobalt dopant, and a vanadium dopant. Claim 15 The electrodeless laser-driven light source according to claim 1, wherein the Q-switching laser crystal comprises a narrowband filter. Claim 16 In claim 15, the narrowband filter reflects at least a portion of the high-intensity light from the gas bulb, an electrodeless laser-driven light source. Claim 17 In claim 15, the narrowband filter is an electrodeless laser-driven light source that blocks wavelengths of the xenon spectrum. Claim 18 In claim 1, the gas-filled bulb is an electrodeless laser-driven light source comprising xenon gas. Claim 19 In claim 1, the gas-filled bulb is an electrodeless laser-driven light source comprising a noble gas. Claim 20 In claim 1, the gas-filled bulb is an electrodeless laser-driven light source formed in a spherical shape. Claim 21 An electrodeless laser-driven light source according to claim 1, wherein the pressure of the gas-filled bulb is in the range of 20 atm to 50 atm. Claim 22 A method for igniting an electrodeless laser-driven light source comprising: a) providing electromagnetic energy to a gas in a gas-filled bulb using continuous CW laser light; b) providing laser pump radiation to a Q-switching laser crystal to generate laser pulses; c) providing said laser pulses to the gas in the gas-filled bulb to generate a pulsed plasma in a breakdown region of said gas; d) generating a CW plasma that emits high-intensity light in a CW plasma region using the electromagnetic energy provided to the ionized gas and said pulsed plasma; and e) passing said laser pump radiation provided to the Q-switching laser crystal through said CW plasma region so as to turn off the pulsed light generated by said Q-switching laser crystal by the CW plasma absorbing the pump light. Claim 23 A method of igniting an electrodeless laser-driven light source according to claim 22, wherein providing electromagnetic energy to the gas in the gas-filled bulb and providing laser pump radiation to the Q-switching laser crystal comprises using the same laser to provide the electromagnetic energy and the pump radiation. Claim 24 A method of igniting an electrodeless laser-driven light source according to claim 22, wherein the breakdown region and the CW plasma region overlap. Claim 25 A method of igniting an electrodeless laser-driven light source according to claim 22, wherein the breakdown region and the CW plasma region do not overlap. Claim 26 A method of igniting an electrodeless laser-driven light source, wherein the pulse repetition rate of the laser pulses is in the range of 1 kHz or less, according to claim 22. Claim 27 A method of igniting an electrodeless laser-driven light source, wherein the pulse repetition rate of the laser pulses is in the range of 1 kHz to 20 kHz. Claim 28 A method of igniting an electrodeless laser-driven light source, wherein the pulse energy of the laser pulses is in the range of 350 μJ to 50 mJ. Claim 29 A method of igniting an electrodeless laser-driven light source according to claim 22, wherein the pulse duration of the laser pulses is in the range of 0.1 ns to 10 ns. Claim 30 A method of igniting an electrodeless laser-driven light source, wherein the power of the continuous CW laser light is in the range of 5 W to 1500 W.

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